Monodisperse core/shell and other complex structured nanocrystals and methods of preparing the same

ABSTRACT

The present invention provides new compositions containing nearly monodisperse colloidal core/shell semiconductor nanocrystals with high photoluminescence quantum yields (PL QY), as well as other complex structured semiconductor nanocrystals. This invention also provides new synthetic methods for preparing these nanocrystals, and new devices comprising these compositions. In addition to core/shell semiconductor nanocrystals, this patent application also provides complex semiconductor nanostructures, quantum shells, quantum wells, doped nanocrystals, and other multiple-shelled semiconductor nanocrystals.

PRIOR RELATED U.S. APPLICATION DATA

This application is a divisional application of U.S. patent applicationSer. No. 10/763,068, filed Jan. 22, 2004, which claims priority to U.S.Patent Application Ser. No. 60/442,146, filed Jan. 22, 2003, which isincorporated by reference herein in its entirety.

STATEMENT OF GOVERNMENT LICENSE RIGHTS

The inventors received partial funding support through the NationalScience Foundation (Grant No. CHE0101178) and support through the jointOklahoma University-University of Arkansas Center for SemiconductorPhysics in Nanostructures. The Federal Government may retain certainlicense rights in this invention.

TECHNICAL FIELD OF THE INVENTION

The present invention relates to semiconductor nanocrystalline materialsand to methods of making and using such materials.

BACKGROUND OF THE INVENTION

Colloidal semiconductor nanocrystals are nanometer-sized fragments ofthe corresponding bulk crystals, which have generated fundamentalinterest for their promise in developing advanced optical materials. Thesize-dependent emission is probably the most attractive property ofsemiconductor nanocrystals, for example, differently sized CdSenanocrystals can be prepared that emit from blue to red, withcomparatively pure color emissions. These nanocrystal-based emitters canbe used for many purposes, such as for light-emitting diodes, lasers,biomedical tags, and the like.

One type of useful nanocrystalline material is the core/shellnanocrystal, which features a nanocrystalline core of one type material,coated with a shell of another type material. Core/shell nanocrystalsare representative of a number of different complex structurednanocrystals, such as core/shell/shell structured materials, thearchitectures of which are aimed at providing fine control over thenanocrystal's photophysical properties. However, synthetic methods forpreparing high quality, nearly monodisperse core/shell and other complexstructured nanocrystals have lagged behind those available for thesynthesis of plain core nanocrystals.

What are needed are improved methods to produce high quality, highlymonodisperse core/shell and other complex structured nanocrystals thatprovide materials that approach the same high quality as the plain corenanocrystals currently available. In particular, synthetic techniquesare needed that allow the preparation of these complex structurednanocrystals to approach the same level of control over fundamentalparameters such as, crystallinity, size and size distribution, as thatattained in the synthesis of plain core nanocrystals. The availabilityof such synthetic methods should improve the quality of known core/shellstructures, and should generate many unexplored nanostructures.

SUMMARY OF THE INVENTION

Core/shell semiconductor nanocrystals, in which the core compositiondiffers from the composition of the shell that surrounds the core, areuseful for many optical applications. If the band offsets of thecore/shell structures are type-I, and the shell semiconductor possessesa higher bandgap than the core material does, then the photo-generatedelectron and hole inside a nanocrystal will be mostly confined withinthe core. As used herein, type-I band offsets refer to a core/shellelectronic structure wherein both conduction and valence bands of theshell semiconductor are simultaneously either higher or lower than thoseof the core semiconductor. Consequently, conventional core/shellnanocrystals can show high photoluminescence (PL) andelectroluminescence efficiencies and can be more stable againstphoto-oxidation than “plain core” semiconductor nanocrystals comprisinga single material, provided that the bandgap of the core semiconductoris smaller than that of the shell semiconductor. See, for example:Hines, M. A.; Guyot-Sionnest, P. J. Phys. Chem. 1996, 100, 468; Peng,X.; Schlamp, M. C.; Kadavanich, A. V.; Alivisatos, A. P. J. Am. Chem.Soc. 1997, 119, 7019-7029; Dabbousi, B. O.; RodriguezViejo, J.; Mikulec,F. V.; Heine, J. R.; Mattoussi, H.; Ober, R.; Jensen, K. F.; Bawendi, M.G. J. Phys. Chem. B 1997, 101, 9463-9475; Micic, O. I.; Smith, B. B.;Nozik, A. J. J. Phys. Chem. B 2000, 104, 12149-12156; Cao, Y.; Banin, U.J. Am. Chem. Soc. 2000, 122, 9692-9702; Manna, L.; Scher, E. C.; Li,L.-S.; Alivisatos, A. P. J. Am. Chem. Soc. 2002, 124, 7136-7145;Cumberland, S. L.; Hanif, K. M.; Javier, A.; Khitrov, G. A.; Strouse, G.F.; Woessner, S. M.; Yun, C. S. Chem. Mater. 2002, 14, 1576-1584; Reiss,P.; Bleuse, J.; Pron, A. Nano Lett. 2002, 2, 781-784; Schlamp, M. C.;Peng, X. G.; Alivisatos, A. P. J. Appl. Phys. 1997, 82, 5837-5842;Mattoussi, H.; Radzilowski, L. H.; Dabbousi, B. O.; Thomas, E. L.;Bawendi, M. G.; Rubner, M. F. J. Appl. Phys. 1998, 83, 7965-7974;Tesster, N.; Medvedev, V.; Kazes, M.; Kan, S.; Banin, U. Science 2002,295, 1506-1508; each of which is incorporated by reference herein, inits entirety.

Recent advancements in synthesis of semiconductor nanocrystals have madeit possible to obtain highly luminescent plain core nanocrystals. (See,for example: Hines, M. A.; Guyot-Sionnest, P. J. Phys. Chem. B 1998,102, 3655-3657; Talapin, D.; Rogach, A. L.; Kornowski, A.; Haase, M.;Weller, H. Nano Lett. 2001, 1, 207; Qu, L.; Peng, X. J. Am. Chem. Soc.2002, 124, 2049-2055; each of which is incorporated by reference herein,in its entirety.) However, plain core nanocrystals may suffer from lackof stability and processability. While not intending to be bound bytheory, it is possible that the underlying causes of this instabilityand poor processability may be intrinsic. Further, plain corenanocrystals may also be chemically and thermally less stable than acorresponding core/shell nanocrystal comprising a core of the samecomposition as the plain core nanocrystal, largely as a result of theirlack of a protective coating of another, more stable, material.Therefore, core/shell nanocrystals are likely to be the desiredstructures when the nanocrystals must either undergo complicatedchemical treatments, such as in bio-medical applications, or when thenanocrystals are constantly excited, such as for LEDs and lasers. (See,for example: Bruchez, M.; Moronne, M.; Gin, P.; Weiss, S.; Alivisatos,A. P. Science 1998, 281, 2013-2016; Chan, W. C. W.; Nie, S. M. Science1998, 281, 2016-2018; Mattoussi, H.; Mauro, J. M.; Goldman, E. R.;Anderson, G. P.; Sundar, V. C.; Mikulec, F. V.; Bawendi, M. G. J. Am.Chem. Soc. 2000, 122, 12142; Han, M.; Gao, X.; Su, J. Z.; Nie, S, Nat.Biotechnol. 2001, 19, 631-635; Schlamp, M. C.; Peng, X. G.; Alivisatos,A. P. J. Appl. Phys. 1997, 82, 5837-5842; Mattoussi, H.; Radzilowski, L.H.; Dabbousi, B. O.; Thomas, E. L.; Bawendi, M. G.; Rubner, M. F. J.Appl. Phys. 1998, 83, 7965-7974; Tesster, N.; Medvedev, V.; Kazes, M.;Kan, S.; Banin, U. Science 2002, 295, 1506-1508; Klimov, V. I.;Mikhailovsky, A. A.; Xu, S.; Malko, A.; Hollingsworth, J. A.;Leatherdale, C. A.; Eisler, H. J.; Bawendi, M. G. Science 2000, 290,314-317; each of which is incorporated by reference herein, in itsentirety.)

The present invention addresses many of the current limitations inobtaining highly monodisperse core/shell nanocrystals, quantum shells,quantum wells, doped nanocrystals, and other complex structurednanocrystals by providing new synthetic strategies, nanocrystallinestructures, compositions, devices, and methods encompassing thesematerials.

In one aspect, for example, this invention provides new compositionscontaining highly monodisperse colloidal core/shell semiconductornanocrystals with high photoluminescence quantum yields (PL QY), as wellas other complex structured semiconductor nanocrystals. In anotheraspect, for example, this invention provides a high level of controlover the thickness of a shell which overcoats a nanocrystalline core,and a high level of control over the thickness of additional shellswhich overcoat any underlying shells. In comparison, the size, sizedistribution, and optical properties of core/shell nanocrystals gownusing existing methods become significantly worse than the core or thecore/shell nanocrystals coated with a thin layer of shell, typically,for example, one to two monolayers. (See, for example: Hines, M. A.;Guyot-Sionnest, P. J. Phys. Chem. 1996, 100, 468; Peng, X.; Schlamp, M.C.; Kadavanich, A. V.; Alivisatos, A. P. J. Am. Chem. Soc. 1997, 119,7019-7029; Dabbousi, B. O.; RodriguezViejo, J.; Mikulec, F. V.; Heine,J. R.; Mattoussi, H.; Ober, R.; Jensen, K. F.; Bawendi, M. G. J. Phys.Chem. B 1997, 101, 9463-9475; Micic, O. I.; Smith, B. B.; Nozik, A. J.J. Phys. Chem. B 2000, 104, 12149-12156; Cao, Y.; Banin, U. J. Am. Chem.Soc. 2000, 122, 9692-9702; Manna, L.; Scher, E. C.; Li, L.-S.;Alivisatos, A. P. J. Am. Chem. Soc. 2002, 124, 7136-7145; Cumberland, S.L.; Hanif, K. M.; Javier, A.; Khitrov, G. A.; Strouse, G. F.; Woessner,S. M.; Yun, C. S. Chem. Mater. 2002, 14, 1576-1584; Reiss, P.; Bleuse,J.; Pron, A. Nano Lett. 2002, 2, 781-784.)

In another aspect, for example, this invention also provides newsynthetic methods for preparing highly monodisperse colloidal core/shellsemiconductor nanocrystals, and new devices comprising thesecompositions. In this aspect, for example, this invention also providesa high level of control over the thickness of a shell which overcoats ananocrystalline core, and a high level of control over the thickness ofadditional shells which overcoat any underlying shells. In addition tocore/shell semiconductor nanocrystals, this patent application alsoprovides complex semiconductor nanostructures, quantum shells, quantumwells, doped nanocrystals, and other multiple-shelled semiconductornanocrystals. This invention, therefore, provides new materials, newmethods, and new devices comprising the new materials of this invention.

In one aspect, the present invention encompasses a method for preparingcore/shell and other complex structured nanocrystals, referred to hereinas either successive ionic layer adsorption and reaction (SILAR), orsolution atomic layer epitaxy (SALE), for the growth of high qualitycore/shell nanocrystals of compound semiconductors. This solution phaseepitaxial method differs, in one aspect, from more traditionalapproaches in that growth of the shell material onto the corenanocrystal was achieved in one monolayer a time, by alternatinginjections of the cationic and anionic solutions into the reactionmixture.

The SILAR method can be applied to a wide range of core/shellcrystalline materials, and can be used to prepare core/multiple shellmaterials, such as core/shell/shell, core/shell/shell/shell,core/shell/shell/shell/shell compositions, and the like. The principlesof SILAR (or SALE) were demonstrated by the preparation of, among otherthings, highly monodisperse CdSe/CdS core/shell nanocrystals, in whichthe shell-thickness-dependent optical properties were used as probes ofthe nanocrystal size and size distribution.

As used herein, the terminology “composition 1/composition 2” isemployed to reflect the composition of the core material (composition 1)and the shell material (composition 2). When more than one shell ispresent over a nanocrystalline core, the terminology “composition1/composition 2/composition 3” is used herein to reflect the compositionof the core material (composition 1), the first shell material(composition 2), and the third shell material (composition 3).Compositions with more than two shell layers are described using asimilar notation method.

In one aspect, the present invention provides nanocrystals comprising avariety of compositions and characterized by a variety of structuralmotifs that exhibit a narrow size distribution, that is, are highlymonodisperse. For example, the narrow size distribution of core/shellnanocrystals, such as CdSe/CdS, CdS/CdSe, or CdS/CdSe/CdS, preparedaccording to this invention was maintained even after at least about 15monolayers of the shell were grown, equivalent to about a forty-foldvolume increase for a 3.5 nm core nanocrystal. The epitaxial growth ofthe core/shell structures was verified by optical spectroscopy,transmission electron microscopy (TEM), and x-ray diffraction (XRD)techniques. The photoluminescence quantum yield (PL QY) of theas-prepared CdSe/CdS core/shell nanocrystals ranged from about 20% toabout 50%. As used herein, the terminology “as-prepared” nanocrystals ornanocrystalline products refer to those nanocrystal samples dissolved inthe original reaction mixture or diluted by a solvent without theremoval of any unreacted reactants and the side products. Further,several types of brightening phenomena were observed for the as-preparedCdSe/CdS core/shell nanocrystals which further boosted theirphotoluminescence quantum yield (PL QY). In addition, the processabilityof the CdSe/CdS core/shell nanocrystals was found to be superior incomparison to the highly luminescent CdSe plain core nanocrystals.

This invention further encompasses the application of SILAR technologyto many other types of core/shell structures, including but not limitedto CdSe/ZnSe, CdSe/ZnS, InP/CdS, InAs/CdS, InAs/InP, InAs/CdSe,CdS/CdSe, CdS/InP, CdS/CdSe/CdS, CdSe/ZnS/CdSe, CdSe/ZnS/CdSe/ZnS,ZnSe/Zn_(x)Cu_(1-x)Se/ZnSe, and the like. The core/shell structures ofthe types CdS/CdSe, CdS/InP, and the like, are examples of a type ofquantum structures termed “quantum shells.” Nanocrystalline CdS/CdSe/CdScore/shell/shell structures are examples of colloidal quantum wells.Nanocrystalline ZnSe/Zn_(x)Cu_(1-x)Se/ZnSe core/shell/shell structuresare examples of doped nanocrystals. Nanocrystalline CdSe/ZnS/CdSe andCdSe/ZnS/CdSe/ZnS structures represent examples of dual emittingnanocrystals. According to the present invention, synthetic methods areprovided for precise control of the radial distribution of the dopants.

Likely due to the large band offsets between the core and shellsemiconductors, the photoluminescence quantum yields (PL QY) of theCdSe/ZnS and CdSe/ZnSe core/shell nanocrystals were found to be evenhigher than that of the CdSe/CdS system. For the core/shell nanocrystalswith III-V semiconductor nanocrystals as the cores, the PL QY andphotochemical stability of the core/shell nanocrystals were found to besignificantly improved as compared to the plain core III-V semiconductornanocrystals.

In one example, the photoluminescence quantum yield (PL QY) of theCdS/CdSe quantum shells according to this invention was found to be ashigh as 20%. The epitaxial growth of an extra shell of CdS onto theCdS/CdSe quantum shells, which formed colloidal quantum wells, furtherenhanced the PL of the CdSe layer to as high as about 50%.

Further, air-stable, relatively inexpensive, and safe precursors andsolvents were found to be compatible with SILAR technology, and thesyntheses could readily be performed on multigram scales. These factorsmade the SILAR technology of this invention very cost effective incomparison to existing synthetic schemes.

In one aspect, the present invention provides a composition comprisingcore/shell nanocrystals, wherein:

the nanocrystals comprise a core material and a shell materialovercoating the core material, each of which is independently selectedfrom a II/VI compound or a III/V compound,

the band gap of the core material is less than the band gap of the shellmaterial; and

the thickness of the shell material is from 1 to about 15 monolayers.

In another aspect, the present invention provides the core/shellnanocrystals themselves.

In another aspect, for example, this invention provides a compositioncomprising nanocrystalline, core/shell quantum shells, wherein:

the quantum shells comprise a core material and a shell materialovercoating the core material;

the core material comprises a stable, nanometer-sized inorganic solid;

the shell material overcoating the core material is selected from aII/VI compound or a III/V compound;

the band gap of the core material is greater than the band gap of theshell material;

the thickness of the shell material is from 1 to about 15 monolayers;and

the as-prepared quantum shells having the shell thickness greater than 1monolayer exhibit a photoluminescence that is substantially limited to abandgap emission, with a photoluminescence quantum yield (PL QY) up toabout 20%.

In another aspect, the present invention provides a compositioncomprising nanocrystalline, core/shell/shell quantum wells, wherein:

the quantum wells comprise a core material, a first shell materialovercoating the core material, and a second shell material overcoatingthe first shell material;

the core material comprises a stable, nanometer-sized inorganic solid;

the first shell material and the second shell material are independentlyselected from a II/VI compound or a III/V compound;

the band gap of the first shell material is less than the band gap ofthe core material and less than the band gap of the second shellmaterial; and

the as-prepared quantum wells exhibit a photoluminescence that issubstantially limited to a bandgap emission, with a photoluminescencequantum yield (PL QY) up to about 50%.

In still another aspect, this invention provides a compositioncomprising nanocrystalline, core/multiple shell quantum wells, wherein:

the quantum wells comprise a core material, a first shell materialovercoating the core material, a second shell material overcoating thefirst shell material, and optionally comprising additional shellmaterials sequentially overcoating underlying shells;

the core material comprises a stable, nanometer-sized inorganic solid;

the first shell material and the second shell material are independentlyselected from a II/VI compound or a III/V compound;

any additional shells are independently selected from a II/VI compoundor a III/V compound; and

the band gap of any shell material is less than the band gap of the bothadjacent core or shell materials, or greater than the band gap of theboth adjacent core or shell materials.

In a further aspect, this invention provides a composition comprisingradially-doped, or simply, “doped”, core/shell/shell nanocrystalswherein:

the radially-doped nanocrystals comprise a core material, a first shellmaterial overcoating the core material, and a second shell materialovercoating the first shell material;

the core material comprises a compound of the formula M¹ _(x)E_(y),wherein M¹ is selected from a metal, E is selected from a non-metal, andx and y are dictated by the stoichiometry of the compound;

the first shell material comprises a compound of the formula M¹ _(x-z)M²_(z)E_(y), wherein M² is selected from a transition metal or a mixturethereof, 0≦z<x, and M² is different than M¹; and

the second shell material comprises a compound of the formula M¹_(x-q)M³ _(q)E_(y), wherein M³ is selected from a transition metal, arare earth metal, or a mixture thereof, 0≦q≦x, and x is not equal to qwhen M² is the same as M³.

In yet another aspect, this invention provides a composition comprisingradially-doped, core/multiple shell nanocrystals wherein:

the radially-doped nanocrystals comprise a core material, a first shellmaterial overcoating the core material, a second shell materialovercoating the first shell material, and a third shell materialovercoating the second shell material;

the core material comprises a compound of the formula M¹ _(a)E¹ _(b),wherein M¹ is selected from a group II or group III metal; E¹ isselected from a non-metal; and a and b are dictated by the stoichiometryof the compound;

the first shell material comprises a compound of the formula M¹ _(a-c)M²_(c)E¹ _(b), wherein M² is selected from at least one transition metal;0≦c<a; and M² is different than M¹;

the second shell material comprises a compound of the formula M³_(d-f)M⁴ _(f)E¹ _(e), wherein M³ is selected from a group II or groupIII metal; M⁴ is selected from a transition metal, a rare earth metal,or a mixture thereof; d and e are dictated by the stoichiometry of thecompound M³ _(d)E³ _(e); and 0≦f<d;

the third shell material comprises a compound of the formula M⁵ _(g-i)M⁶_(i)E⁵ _(h), wherein M⁵ is selected from a group II or group III metal;M⁶ is selected from a transition metal, a rare earth metal, or a mixturethereof; g and h are dictated by the stoichiometry of the compound M⁵_(g)E⁵ _(h); and 0≦i<g;

wherein the bandgap of the third shell material is greater than thebandgap of the core material, greater than the bandgap of the firstshell material, and greater than the bandgap of the second shellmaterials; and

wherein the thicknesses of the first shell material, the second shellmaterial, and the third shell material are independently varied between0 and 15 monolayers. In another aspect related to these radially-dopedcore/multiple shell nanocrystals: a) i) M¹ can be selected from Zn, Cd,or Hg, and E¹ is selected from O, S. Se, or Te; or

-   -   ii) M¹ can be selected from Ga and In, and E¹ is selected from        N, P and As; and

b) M² can be selected from Mn, Fe, Co, Ni, Pd, Pt, Cu, Al, Ag, or Au, ora rare earth metal.

In yet another aspect, related to these radially-doped core/multipleshell nanocrystals, M¹ _(a)E¹ _(b) can be selected from CdSe, CdS, CdTe,ZnS, ZnSe, ZnTe, HgS, HgSe, HgTe, InAs, InP, GaAs, GaP, ZnO, CdO, HgO,In₂O₃, TiO₂, or a rare earth oxide.

In another aspect related to these radially-doped core/multiple shellnanocrystals, the radially-doped, core/multiple shell nanocrystals cancomprise ZnSe/Zn_(a-c)M² _(c)Se/ZnSe, ZnSe/Zn_(a-c)M² _(c)Se/ZnS,ZnO/Zn_(a-c)M² _(c)O/ZnO, ZnO/Zn_(a-c)M² _(c)O/ZnS, TiO₂/Ti_(a-c)M²_(c)O₂/TiO₂, and wherein M² is selected from Mn, Fe, Co, Ni, Pd, Pt, Al,Cu, Ag, or Au, or a rare earth metal.

In still another aspect, this invention provides a compositioncomprising core/multiple shell nanocrystals which are optionally dopedat the core and optionally doped at any shell. In this aspect, thepresent invention provides a composition comprising radially-doped,core/multiple shell nanocrystals, comprising:

1) a core material having the formula M¹ _(a-c)M² _(c)E¹ _(b), wherein:

-   -   a) M¹ is selected from a metal, E¹ is selected from a non-metal,        and a and b are dictated by the stoichiometry of the compound M¹        _(a)E¹ _(b);    -   b) M² is selected from a transition metal, a rare earth metal,        or a mixture thereof; and M² is different than M¹; and    -   c) 0≦c<a;

2) an optional first shell material overcoating the core material,having the formula M³ _(d-f)M⁴ _(f)E³ _(e), wherein:

-   -   a) M³ is selected from a metal, E³ is selected from a non-metal,        and d and e are dictated by the stoichiometry of the compound M³        _(d)E³ _(e);    -   b) M⁴ is selected from a transition metal, a rare earth metal,        or a mixture thereof; and M⁴ is different than M³; and    -   c) 0≦f<d;

3) an optional second shell material overcoating the optional firstshell material, having the formula M⁵ _(g-i)M⁶ _(i)E⁵ _(h), wherein:

-   -   a) M⁵ is selected from a metal, E⁵ is selected from a non-metal,        and g and h are dictated by the stoichiometry of the compound M⁵        _(g)E⁵ _(h);    -   b) M⁶ is selected from a transition metal, a rare earth metal,        or a mixture thereof; and M⁶ is different than M⁵; and    -   c) 0≦i<g;

4) an optional third shell material overcoating the optional secondshell material, having the formula M⁷ _(j-l)M⁸ _(l)E⁷ _(k), wherein:

-   -   a) M⁷ is selected from a metal, E⁷ is selected from a non-metal,        and j and k are dictated by the stoichiometry of the compound M⁷        _(j)E⁷ _(k);    -   b) M⁸ is selected from a transition metal, a rare earth metal,        or a mixture thereof; and M⁸ is different than M⁷; and    -   c) 0≦l<j; and

5) an optional fourth shell material overcoating the optional thirdshell material, having the formula M⁹ _(m-o)M¹⁰ _(o)E⁹ _(n), wherein:

-   -   a) M⁹ is selected from a metal, E⁹ is selected from a non-metal,        and m and n are dictated by the stoichiometry of the compound M⁹        _(m)E⁹ _(n);    -   b) M¹⁰ is selected from a transition metal, a rare earth metal,        or a mixture thereof; and M¹⁰ is different than M⁹; and    -   c) 0≦o<m.

In another aspect, this invention also provides a composition comprisingcore/shell/shell dual-emitting nanocrystals, wherein:

the nanocrystals comprise a core material, a first shell materialovercoating the core material, and a second shell material overcoatingthe first shell material, each of which is independently selected from aII/VI compound or a III/V compound;

the band gap of the first shell material is greater than the band gap ofthe core material and greater than the band gap of the second shellmaterial; and

the as-prepared dual-emitting nanocrystals exhibit a photoluminescencecomprising bandgap emission peaks.

In this aspect, the additional shells are independently selected fromselected from a II/VI compound or a III/V compound; and the band gap ofthe additional shell material is greater than the band gap of the secondshell material.

In yet another aspect, this invention provides a composition comprisingcore/shell/shell/shell dual-emitting nanocrystals, wherein:

the nanocrystals comprise a core material, a first shell materialovercoating the core material, a second shell material overcoating thefirst shell material, and a third shell material overcoating the secondshell material, each of which is independently selected from a II/VIcompound or a III/V compound;

the band gap of the first shell material and the band gap of the thirdshell material are less than the band gap of the core material and areless than the band gap of the second shell material; and

the as-prepared dual-emitting nanocrystals exhibit a photoluminescencecomprising bandgap emissions.

In one aspect, the dual-emitting nanocrystals can comprise up to atleast about 12 to 15 additional shells, wherein the additional shellsare typically independently selected from a II/VI compound or a III/Vcompound; and wherein the band gap of the additional shell material isgreater than the band gap of the second shell material.

In another aspect, this invention also encompasses a method forpreparing core/shell nanocrystals having the formula M¹X¹/M²X²,comprising:

a) providing a solution of core nanocrystals of the formula M¹X¹;

b) forming a first monolayer of a shell material M²X² by contacting thecore nanocrystals, in an alternating manner, with a cation (M²)precursor solution in an amount effective to form a monolayer of thecation, and an anion (X²) precursor solution in an amount effective toform a monolayer of the anion; and

c) optionally forming subsequent monolayers of shell material M²X² bycontacting the core/shell nanocrystals, in an alternating manner, with acation (M²) precursor solution in an amount effective to form amonolayer of the cation, and an anion (X²) precursor solution in anamount effective to form a monolayer of the anion;

wherein M¹X¹ comprises a stable, nanometer-sized inorganic solid;

wherein M²X² is selected from a II/VI compound or a III/V compound; and

wherein M¹X¹ and M²X² are different. In this aspect, the cationprecursor solution can optionally contain at least one ligand, and theanion precursor solution can optionally contain at least one ligand.

In still another aspect, this invention encompasses a method forpreparing core/shell/shell nanocrystals having the formulaM¹X¹/M²X²/M³X³ comprising:

a) providing a solution of core nanocrystals of the formula M¹X¹;

b) forming at least one monolayer of a first shell material M²X² bycontacting the core nanocrystals, in an alternating manner, with a firstcation (M²) precursor solution in an amount effective to form amonolayer of the first cation, and a first anion (X²) precursor solutionin an amount effective to form a monolayer of the first anion; and

c) forming at least one monolayer of a second shell material M³X³ bycontacting the core nanocrystals, in an alternating manner, with asecond cation (M³) precursor solution in an amount effective to form amonolayer of the second cation, and an second anion (X³) precursorsolution in an amount effective to form a monolayer of the first anion;

wherein M¹X¹ comprises a stable, nanometer-sized inorganic solid;

wherein M²X² and M³X³ are independently selected from a II/VI compoundor a III/V compound; and

wherein M¹X¹, M²X², and M³X³ are different.

In yet another aspect, the present invention provides a method forpreparing radially-doped core/shell/shell nanocrystals having theformula M¹ _(x)E_(y)/M¹ _(x-z)M² _(z)E_(y)/M¹ _(x-q)M³ _(q)E_(y),comprising:

a) providing a solution of core nanocrystals of the formula M¹_(x)E_(y), wherein M¹ is selected from a metal, E is selected from anon-metal, and x and y are dictated by the stoichiometry of thecompound;

b) forming at least one monolayer of a doped first shell material of theformula M¹ _(x-z)M² _(z)E_(y) by contacting the core nanocrystals, in analternating manner, with a cation precursor solution in an amounteffective to form a monolayer of the first cation doped with the secondcation, and a first anion (X²) precursor solution in an amount effectiveto form a monolayer of the first anion;

-   -   wherein the cation precursor solution comprises a first cation        (M¹) precursor, a second cation (M²) precursor, or a combination        thereof, and    -   wherein M² is selected from a transition metal or a mixture        thereof, 0≦z<x, and M² is different than M¹;

c) forming at least one monolayer of a second shell material of theformula M¹ _(x-q)M³ _(q)E_(y) by contacting the core/shell nanocrystals,in an alternating manner, with a first cation precursor solution in anamount effective to form a monolayer of the first cation, and an secondanion (X³) precursor solution in an amount effective to form a monolayerof the first anion;

-   -   wherein the first cation precursor solution optionally comprises        a third cation (M³) precursor selected from a transition metal,        a rare earth metal, or a mixture thereof; and    -   wherein 0≦q≦x, and x is not equal to q when M² is the same as        M³; and

d) optionally repeating steps b and c to form additional shellsovercoating the second shell.

Accordingly, it is one aspect of the present invention to provide newsynthetic methods for preparing semiconductor core/shell and othercomplex structured nanocrystals that are characterized by highphotoluminescence quantum yields.

It is another aspect of this invention is the development of newsynthetic methods for preparing semiconductor core/shell and othercomplex structured nanocrystals that are nearly monodisperse.

It is a further aspect of this invention to provide a method for controlover the stability of the photoluminescence emission of as-preparedsemiconductor core/shell and other complex structured nanocrystals.

In one aspect, for example, this invention provides techniques for thegrowth of high quality core/shell nanocrystals, which may also beamenable for the growth of nanocrystals with very complex compositionprofile along the radial direction of the nanocrystals. For example, acolloidal quantum well can comprise a low bandgap semiconductorsandwiched between two high bandgap semiconductors, epitaxially grown onthe surface of bulk single crystal substrate by molecular beam epitaxyor related techniques. Such structures are of great interest for theirphotophysical properties.

In another aspect, for example, the present invention provides methodsfor synthesizing complex structured doped nanocrystals, wherein thedoping is effected in a given radial position. These “radially-doped”nanocrystalline materials are of interest for, among other things, forspintronics, atomic emitting materials, and the like. Thus, the presentinvention provides, in this aspect, a method for doping a nanocrystal ina given radial position in a controlled fashion.

In yet another aspect, the present invention provides synthetic methodsfor preparing high quality, highly monodisperse core/shell and othercomplex structured nanocrystals. In this aspect, for example, themethods of this invention allow a narrow size distribution of core/shellnanocrystals to be attained as the growth of a shell material proceeds.

The synthetic methods of the present invention provide for preparinghigh quality, nearly monodisperse core/shell and other complexstructured nanocrystals. In this aspect, for example, the methods ofthis invention allow a narrow size distribution of core/shellnanocrystals to be attained as the growth of a shell material proceeds.While not intending to be bound by theory, it is believed that such afeature has been possible by, among other things, the substantialreduction or elimination of any homogeneous nucleation of the shellmaterial that would otherwise provide a mixture of nanocrystals of corematerial and nanocrystals of shell material. In this aspect, essentiallyall the shell precursor material added into a solution of the corenanocrystals grows exclusively onto the core nanocrystals. Again, whilenot intending to be bound by theory, it is believed that nearlymonodisperse core/shell and other complex structured nanocrystals havealso been possible by, among other things, shell growth that not onlyproceeds at substantially the same rate, but also initiates atsubstantially the same time at each nanocrystalline core site.

Another aspect of this invention is the development of methods tocorrelate reaction conditions with the resulting PL QY of semiconductorcore/shell nanocrystals, and to thereby impart control over nanocrystalquantum yield.

It is a further aspect of this invention to provide a method for controlover the stability of the photoluminescence emission of as-preparedsemiconductor core/shell nanocrystals.

Accordingly, it is one aspect of the present invention to provide newsynthetic methods for preparing II-VI, III-V, and other types ofsemiconductor core/shell nanocrystals that are both nanometer size andhighly monodisperse.

It is a further aspect of this invention to provide a method forsynthesizing highly monodisperse, semiconductor core/shell nanocrystalsutilizing inexpensive, low or limited toxicity precursors materials.

Yet another aspect of the present invention is the development of amethod of synthesizing monodisperse semiconductor core/shellnanocrystals using non-coordinating solvents.

A further aspect of this invention is the development of a procedure forcontrolling the average size of the semiconductor core/shellnanocrystals prepared using non-coordinating solvents.

Another aspect of the present invention is the development of methodsfor synthesizing monodisperse core/shell nanocrystals comprisingCdSe/CdS, CdSe/ZnSe, CdSe/ZnS, InP/CdS, InAs/CdS, InAs/InP, InAs/CdSe,CdS/CdSe, CdS/InP, and the like, with high quality and improvedphotoluminescence properties.

Still a further aspect of this invention is the synthesis of a new classof colloidal quantum structures referred to herein as “quantum shells,”characterized by a large energy band gap offset between the core andshell materials, that is quantum shells are characterized by a largeband gap for the core and a small band gap for the shell where thedifference between these band gaps is substantial.

A further aspect of the present invention is the development ofsuccessive ionic layer adsorption and reaction (SILAR) methods, orsolution atomic layer epitaxy (SALE) methods, for the growth of highquality core/shell nanocrystals of compound semiconductors, includingquantum shells.

Still another aspect of this invention is the development of syntheticprocedures for preparing core/shell and other complex structurednanocrystals that allow more environmentally innocuous precursors,ligands, and solvents to be employed, and that are convenient, lessexpensive, safer, faster, and more environmentally “green” than methodscurrently used.

This invention further provides, in one aspect, articles of manufacturecomprising the nanocrystalline materials disclosed herein.

These and other features, aspects, objects and advantages of the presentinvention will become apparent after a review of the following detaileddescription of the disclosed embodiments and appended claims, inconjunction with the drawings described as follows.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 illustrates the following. Top: The accumulative value of the redshift of the first absorption peak for CdSe/CdS nanocrystals due to thegrowth of CdS shell after each injection at different reactiontemperatures. Middle: Formation of isolated CdS nanocrystals atrelatively low temperatures, evidenced by the extremely high absorbanceat high energy part even with thin CdS shell. Bottom: PLE(photoluminescence excitation) spectra of CdSe/CdS nanocrystals beforeand after the removal of isolated CdS particles.

FIG. 2 provides TEM (transmission electron microscopy) images of CdSeplain core nanocrystals and the corresponding core/shell nanocrystalswith different shell thickness from the same SILAR reaction.

FIG. 3 illustrates core/shell nanocrystals with a relatively thick CdSshell, five monolayers in this case, formed superlattice only if theparticle concentration of the deposition solution was high (left). Theselected area electron diffraction (SAED) patterns and the correspondingdiffraction intensity profiles of randomly deposited (top raw) andoriented (bottom raw) core/shell nanocrystals.

FIG. 4 demonstrates HRTEM (high resolution transmission electronmicroscopy) images of four representative core/shell nanocrystals, eachhaving a 3.5 nm CdSe core with five monolayers of CdS shell materialovercoating the CdSe core.

FIG. 5 presents the XRD (X-ray diffraction) pattern of CdSe/CdScore/shell nanocrystals having a 3.5 nm CdSe core with five monolayersof CdS shell material overcoating the CdSe core. For comparison, thestandard powder diffraction patterns of wurtzite CdSe and CdS bulkcrystals are provided.

FIG. 6 illustrates the following. Left: the evolution of the UV-Vis andPL (photoluminescence) spectra of the CdSe/CdS core/shell nanocrystalsupon the growth of the CdS shell in a typical reaction. Right:Asymmetric PL spectra of CdSe/CdS core/shell nanocrystals with fivemonolayers of CdS shell overcoating the CdSe core.

FIG. 7 plots the PL QY (photoluminescence quantum yield) and PL FWHM(photoluminescence emission line full-width at half-maximum intensity)of the as-prepared CdSe/CdS core/shell nanocrystals plotted relative tothe number of CdS monolayers overcoating the CdSe core.

FIG. 8 demonstrates the PL darkening and brightening phenomena observedin the growth and manipulation of CdSe/CdS core/shell nanocrystals.

FIG. 9 presents a comparison of the processability of plain core andcore/shell nanocrystals. Top-right: PL spectra of CdSe plain core andCdSe/CdS core/shell nanocrystals, before and after purification by theprecipitation/decantation procedure. Top-left: PL spectra of the CdSeplain core and CdSe/CdS core/shell nanocrystals upon the deposition ofthe nanocrystals onto substrates. Bottom: UV-Vis and PL spectra of plaincore and core/shell nanocrystals coated with hydrophilic thiols.

FIG. 10 presents a plot of the temporal evolution of UV-Vis of InAs/CdSecore/shell nanocrystals, illustrating, among other things, how thegrowth of the CdSe shell layers shifted the UV-Vis peak of the originalInAs plain core nanocrystals significantly.

FIG. 11 presents a plot of the PL spectra of InAs and InAs/CdSecore/shell nanocrystals, illustrating, among other things, how thegrowth of the CdSe shell layers enhanced the photoluminescence of theInAs plain core nanocrystals significantly.

FIG. 12 provides a plot of the UV-Vis spectra of a series of CdS/CdSequantum shells, which were examined after each sequential layer of CdSewas grown onto the plain CdS core, using SILAR methods.

FIG. 13 presents an illustration of the PL Spectra of CdS/CdSe quantumshells which were examined after each sequential layer of CdSe was grownonto the plain CdS core, using SILAR methods, illustrating, among otherthings, the shift in wavelength.

FIG. 14 provides an illustration of the layer-dependent molar extinctioncoefficient at the first absorption state for CdS/CdSe quantum shells.Quantum shells are shown to possess significantly lower molar extinctioncoefficients in comparison to the corresponding quantum dots. Thisfeature, coupled with the large ensemble Stocks shift dramaticallylowers the re-absorption of their photo- and electro-luminescence.Accordingly, the quantum shell nanocrystals are excellent emitters,especially when a high density of nanocrystals are needed forapplications such as lasers and LEDs.

FIG. 15 illustrates the PLE (photoluminescence excitation) spectra oftwo samples of CdS/CdSe quantum shells with the same shell thickness(five monolayers) but different size cores, illustrating the similarityof the electronic energy states of these four systems, and suggesting a1D confinement feature.

FIG. 16 presents extinction coefficients of various nanocrystals as afunction of nanocrystal size.

FIG. 17 illustrates the optical properties of CdS/CdSe/CdS quantumwells. For comparison, optical properties of the corresponding CdS/CdSequantum shells are also illustrated at the top of this Figure.

FIG. 18 illustrates the optical properties of dual-emittingCdSe/ZnS/CdSe nanocrystals, a type of 0D-2D hybrid nanocrystals. Referto the schematic energy band structure shown on the right. Thephotoluminescence excitation (PLE) spectra shown here reveal that theemission at the low energy resembles that of zero-dimension (0D)nanocrystals, which is consistent with the core emission. Similarly, thePLE of the emission peak at the energy side resembles that of quantumshells (FIG. 15), indicating it is indeed from the quantum shell.

DETAILED DESCRIPTION OF THE INVENTION

The present invention addresses many of the current limitations incore/shell and related complex structured nanocrystals by providing newcompositions, new methods, and new devices based on a syntheticstrategy, termed “successive ionic layer adsorption and reaction”(SILAR) or “solution atomic layer epitaxy” (SALE), for the growth ofhigh quality, highly monodisperse core/shell and other complexstructured nanocrystals of compound semiconductors. This solution phaseepitaxial method achieves growth of the shell material onto the corematerial typically one monolayer per reaction cycle, by alternatinginjections of the cationic and anionic solutions into the reactionmixture. One reaction cycle refers to two consecutive injections of thereaction precursors, one for the cationic (anionic) precursor solutionand the following one for the anionic (cationic) precursor solution.

The Successive Ionic Layer Adsorption and Reaction (SILAR) Method andthe Preparation and Properties of Core/Shell Nanocrystals Prepared UsingSILAR

In one embodiment, the present invention comprises core/shellnanocrystals and methods of preparing core/shell nanocrystals. Inseveral examples, the CdSe/CdS core/shell nanocrystals are used asexemplary core/shell nanocrystals of this method.

It was discovered that although 1-octadecene (ODE) is a non-coordinatingsolvent, it provided good solubility for the core nanocrystals of CdSe,and for the two injection solutions comprising the cation precursor,cadmium oxide, and the anion precursor, elemental sulfur. Further, ODEhas a large liquid range, relatively low cost, low toxicity, and lowreactivity to precursors, thereby providing a more environmentally“green” synthesis than previous core/shell nanocrystal syntheses.

Relatively safe and inexpensive precursors for the growth of the CdSshell onto the CdSe core nanocrystals could also be used. In one aspect,this invention provides a method of preparing CdSe/CdS nanocrystalswherein the cation precursor was selected as CdO and the anion precursorwas selected as elemental sulfur. In one aspect, for example, theinjection solution of Cd was prepared by dissolving CdO in ODE usingfatty acids, including but not limited to, stearic acid, oleic acid, anddodecanoic acid, as the ligands. Elemental sulfur could be dissolved inwarm ODE directly, and the resulting solution was used as the sulfurinjection solution. In this aspect, octadecylamine (ODA) was used as aligand for the core/shell nanocrystals.

In one aspect, Example 1 illustrates a preparative method of thisinvention for the synthesis of the core CdSe nanocrystals, in which theCdSe nanocrystals were stabilized in solution by coating then with ODAligands. In comparison to the TOPO (trioctylphosphine)-coatednanocrystals, these amine-coated cores yielded core/shell nanocrystalswith significantly higher PL QY. (See, “Control of PhotoluminescenceProperties of CdSe Nanocrystals in Growth”, Qu L., Peng, X., J. Am.Chem. Soc., 2002, vol 124, p 2049, which is incorporated herein byreference in its entirety.)

Example 2 provides synthesis of the cadmium oxide (cation precursor) andsulfur (anion precursor) injection solutions, Example 3 describes thecalculations performed in determining the injection procedure, includingthe amount of solution to be injected, using the two injectionsolutions.

Two loading methods were tested for adding the CdSe core nanocrystalsinto the reaction flask. The first loading method comprised loading thenanocrystals as a solution in hexanes, in which the nanocrystals hadnever been precipitated out of solution after their preparation. In thisaspect, the purification of the core nanocrystals was performed by anextraction method described in Example 1. The second loading methodcomprised loading the nanocrystals in which the core nanocrystals hadbeen purified by precipitation, followed by decantation of thesupernatant, after which the purified precipitate was loaded into thereaction mixture. In this aspect, the first, solution-based loadingapproach generally provided better growth of high quality core/shellnanocrystals than the second method. For example, transmission electronmicroscopy (TEM) measurements revealed that aggregates and fusedparticles were formed by precipitation of the amine-coated CdSenanocrystals by the second method. Although this invention encompassesnanocrystals prepared by either approach, the results disclosed hereinfor the core/shells using CdSe nanocrystal cores were obtained using thesolution-based loading method.

Example 4 describes one aspect of the preparative method of thisinvention for the synthesis of CdSe/CdS core/shell nanocrystals usingsuccessive ionic layer adsorption and reaction (SILAR). Example 5provides an example of how the SALE method may be used to preparemultigram quantities of the CdSe/CdS core/shell nanocrystals.

In one aspect of this invention, there were typically two methods bywhich SILAR methodology was used to prepare colloidal core/shellnanocrystals of the present invention. As disclosed herein, the core wascoated or contacted with a first shell monolayer by alternatelycontacting the core nanocrystal with a cation precursor solution and ananion precursor solution, in which both cation and anion precursorsolution were employed in an amount sufficient to form one monolayer ofthe shell material. This method was useful when monolayer accuracy wasdesired. However, in another aspect, it is also possible to grow a shelllayer with less than one monolayer thickness by adding less precursorsin one injection cycle. In addition, when monolayer accuracy was notrequired, multiple shell monolayers were coated onto a core byalternately contacting the core nanocrystal with a cation precursorsolution in an amount sufficient to form the desired shell thickness,and contacting the core nanocrystal with an anion precursor solution inan amount sufficient to form the desired shell thickness. In this case,multiple monolayers of shell could be conveniently added.

The reaction temperature for the growth of the CdS shell onto CdSenanocrystals through SILAR in ODE was investigated. FIG. 1 (top plot)illustrates the evolution of the UV-Vis peak position of the core/shellnanocrystals, representing the change in shell thickness since anidentical core sample was used in each plot, after each injection ofeither cadmium or sulfur solution at different reaction temperatures.These data revealed that the accumulated thickness of the shell aftereach injection decreased systematically as the reaction temperaturedecreased. Further, the influence of temperature on the cadmiuminjections was more pronounced than for the sulfur injections (FIG. 1,top plot). In this aspect, for example, for preparation of the CdSe/CdScore/shell nanocrystals using the methods and precursors disclosedherein, the SILAR shell-growth works well at temperatures from about220° C. to about 250° C. In another aspect, the CdSe/CdS core/shellnanocrystals underwent SILAR shell-growth from about 220° C. to about250° C., and in another aspect, from about 235° C. to about 245° C.

Thus, in one aspect, the present invention provides a method forpreparing core/shell nanocrystals having the formula M¹X¹/M²X²,comprising:

a) providing a solution of core nanocrystals of the formula M¹X¹;

b) forming a first monolayer of a shell material M²X² by contacting thecore nanocrystals, in an alternating manner, with a cation (M²)precursor solution in an amount effective to form a monolayer of thecation, and an anion (X²) precursor solution in an amount effective toform a monolayer of the anion; and

c) optionally forming subsequent monolayers of shell material M²X² bycontacting the core/shell nanocrystals, in an alternating manner, with acation (M²) precursor solution in an amount effective to form amonolayer of the cation, and an anion (X²) precursor solution in anamount effective to form a monolayer of the anion;

wherein M¹X¹ comprises a stable, nanometer-sized inorganic solid;

wherein M²X² is selected from a II/VI compound or a III/V compound; and

wherein M¹X¹ and M²X² are different. In this aspect, the cationprecursor solution can optionally contain at least one ligand, and theanion precursor solution can optionally contain at least one ligand.

In another aspect, the present invention provides a method for preparingcore/shell nanocrystals having the formula M¹X¹/M²X², comprising:

a) providing a solution of core nanocrystals of the formula M¹X¹;

b) forming at least one monolayer of a shell material M²X² by contactingthe core nanocrystals, in an alternating manner, with a cation (M²)precursor solution in an amount effective to form a monolayer of thecation, and an anion (X²) precursor solution in an amount effective toform a monolayer of the anion;

wherein M¹X¹ comprises a stable, nanometer-sized inorganic solid;

wherein M²X² is selected from a II/VI compound or a III/V compound; and

wherein M¹X¹ and M²X² are different. Also in this aspect, the cationprecursor solution can optionally contain at least one ligand, and theanion precursor solution can optionally contain at least one ligand.

In another aspect, M¹X¹ can comprise a II/VI compound or a III/Vcompound, and in yet another aspect of this invention, M¹X¹ and M²X² canbe independently selected from CdS, CdSe, CdTe, ZnS, ZnSe, ZnTe, HgS,HgSe, HgTe, ZnO, CdO, InP, InAs, GaAs, or GaP.

In still another aspect, this invention provides a compositioncomprising core/shell nanocrystals, wherein the core material isselected from CdSe, CdS, CdTe, ZnSe, ZnS, ZnTe, HgSe, HgS, HgTe, ZnO,CdO, GaAs, InAs, GaP, or InP;

the shell material is selected from CdSe, CdS, CdTe, ZnSe, ZnS, ZnTe,HgSe, HgS, HgTe, ZnO, CdO, GaAs, InAs, GaP, or InP; and

the shell material is different from the core material.

In another aspect of this invention, the SILAR method described hereinprovides a method whereby the cation (M²) precursor solution can becontacted with the core nanocrystals before the anion (X²) precursorsolution can be contacted with the core nanocrystals. In another aspect,the SILAR method described herein provides a method whereby the anion(X²) precursor solution can be contacted with the core nanocrystalsbefore the cation (M²) precursor solution can be contacted with the corenanocrystals.

Yet another aspect of the SILAR method is the further purification ofthe core/shell nanocrystals.

In still another aspect, the cation precursor solution of this inventioncan comprise, for example, a metal oxide, a metal halide, a metalnitride, a metal ammonia complex, a metal amine, a metal amide, a metalimide, a metal carboxylate, a metal acetylacetonate, a metal dithiolate,a metal carbonyl, a metal cyanide, a metal isocyanide, a metal nitrile,a metal peroxide, a metal hydroxide, a metal hydride, a metal ethercomplex, a metal diether complex, a metal triether complex, a metalcarbonate, a metal phosphate, a metal nitrate, a metal nitrite, a metalsulfate, a metal alkoxide, a metal siloxide, a metal thiolate, a metaldithiolate, a metal disulfide, a metal carbamate, a metaldialkylcarbamate, a metal pyridine complex, a metal bipyridine complex,a metal phenanthroline complex, a metal terpyridine complex, a metaldiamine complex, a metal triamine complex, a metal diimine, a metalpyridine diimine, a metal pyrazolylborate, a metal bis(pyrazolyl)borate,a metal tris(pyrazolyl)borate, a metal nitrosyl, a metal thiocarbamate,a metal diazabutadiene, a metal dithiocarbamate, a metaldialkylacetamide, a metal dialkylformamide, a metal formamidinate, ametal phosphine complex, a metal arsine complex, a metal diphosphinecomplex, a metal diarsine complex, a metal oxalate, a metal imidazole, ametal pyrazolate, a metal-Schiff base complex, a metal porphyrin, ametal phthalocyanine, a metal subphthalocyanine, a metal picolinate, ametal piperidine complex, a metal pyrazolyl, a metal salicylaldehyde, ametal ethylenediamine, a metal triflate compound, or any combinationthereof.

In still another aspect of this invention, for example, the cationprecursor solution can comprise, for example, a metal oxide, a metalcarbonate, a metal bicarbonate, a metal sulfate, a metal sulfite, ametal phosphate, metal phosphite, a metal halide, a metal carboxylate, ametal hydroxide, a metal alkoxide, a metal thiolate, a metal amide, ametal imide, a metal alkyl, a metal aryl, a metal coordination complex,a metal solvate, a metal salt, or a combination thereof.

Yet another aspect of this invention is a cation precursor solutioncomprising a ligand selected from a fatty acid, an fatty amine, aphosphine, a phosphine oxide, a phosphonic acid, a phosphinic acid, asulphonic acid, or any combination thereof, any one of which having upto about 30 carbon atoms. In another aspect, the cation precursorsolution comprising a ligand selected from a fatty acid, an fatty amine,a phosphine, a phosphine oxide, a phosphonic acid, a phosphinic acid, asulphonic acid, or any combination thereof, any one of which having upto about 45 carbon atoms.

Still another aspect of this invention is an anion precursor comprisingan element, a covalent compound, an ionic compound, or a combinationthereof.

Similarly, in another aspect, this invention provides, for example, amethod for preparing core/shell/shell nanocrystals having the formulaM¹X¹/M²X²/M³X³ comprising:

a) providing a solution of core nanocrystals of the formula M¹X¹;

b) forming at least one monolayer of a first shell material M²X² bycontacting the core nanocrystals, in an alternating manner, with a firstcation (M²) precursor solution in an amount effective to form amonolayer of the first cation, and a first anion (X²) precursor solutionin an amount effective to form a monolayer of the first anion; and

c) forming at least one monolayer of a second shell material M³X³ bycontacting the core nanocrystals, in an alternating manner, with asecond cation (M³) precursor solution in an amount effective to form amonolayer of the second cation, and an second anion (X³) precursorsolution in an amount effective to form a monolayer of the first anion;

wherein M¹X¹ comprises a stable, nanometer-sized inorganic solid;

wherein M²X² and M³X³ are independently selected from a II/VI compoundor a III/V compound; and

wherein M¹X¹, M²X², and M³X³ are different.

In this aspect, the cation precursors, the cation precursor solutions,the anion precursors, the anion precursor solutions, the ligands, andthe like, can comprise those elements, compounds, and materialsdisclosed herein.

Further, in this aspect, M¹X¹ can comprises a II/VI compound or a III/Vcompound. In yet another aspect of this invention, M¹X¹, M²X², and M³X³can be independently selected from CdS, CdSe, CdTe, ZnS, ZnSe, ZnTe,HgS, HgSe, HgTe, ZnO, CdO, InP, InAs, GaAs, or GaP.

In another aspect of this invention, the SILAR method described hereinprovides a method whereby the cation (M²) precursor solution iscontacted with the core nanocrystals before the anion (X²) precursorsolution is contacted with the core nanocrystals. In another aspect, theSILAR method described herein provides a method whereby the anion (X²)precursor solution is contacted with the core nanocrystals before thecation (M²) precursor solution is contacted with the core nanocrystals.

Yet another aspect of the SILAR method is the further purification ofthe core/shell nanocrystals.

The cation precursors used in the methods disclosed herein can compriseelements or compounds, for example, elements, covalent compounds, orionic compounds, including but not limited to, oxides, hydroxides,coordination complexes or metal salts, that serve as a source for theelectropositive element or elements in the resulting nanocrystal core orshell material. In one aspect of this invention, inexpensive and safecompounds may be used as cation precursors. Anion precursors can alsocomprise elements, covalent compounds, or ionic compounds that serve asa source for the electronegative element or elements in the resultingnanocrystal. In one aspect of this invention, inexpensive and safeelements or compounds may be used as anion precursors, such as naturallyoccurring substances. These definitions anticipate that ternarycompounds, quaternary compounds, and even more complex species may beprepared using the methods disclosed herein, in which case more than onecation precursor, more than one anion precursor, or both, may be used.When dual or multiple cation elements were used in the growth of a givenmonolayer, the resulting nanocrystals were cation-doped at the givenmonolayer if the other part of the nanocrystals contained only a singlecation. The same method can be used for the preparation of anion-dopednanocrystals.

In one aspect of this invention, the methods disclosed herein areapplicable to core/shell nanocrystals prepared using a range of cationprecursor compounds for the core and the shell material, for example,precursors of the group II metals (for example, Zn, Cd, or Hg), thegroup III metals (for example, Al, Ga, or In), the group IV metals (forexample, Ge, Sn or Pb), or the transition metals (including, but notlimited to, Ti, Zr, Hf, V, Nb, Ta, Cr, Mo, W, Mn, Tc, Re, Fe, Ru, Os,Co, Rh, Ir, Ni, Pd, Pt, Cu, Ag, Au, and the like). (See, F. A. Cotton etal., Advanced Inorganic Chemistry, 6th Edition, (1999).) The cationprecursor for the core and the shell material can constitute a widerange of substances, including, but not limited to, a metal oxide, ametal hydroxide, a metal carbonate, a metal bicarbonate, a metalsulfate, a metal sulfite, a metal phosphate, metal phosphite, a metalhalide, a metal carboxylate, a metal alkoxide, a metal thiolate, a metalamide, a metal imide, a metal alkyl, a metal aryl, a metal coordinationcomplex, a metal solvate, a metal salt, and the like.

In another aspect, ligands, sometimes used for cation precursors, may beselected from a range of compounds including, but not limited to, fattyacids, fatty amines, phosphines, phosphine oxides, phosphonic acids,phosphinic acids, sulphonic acids, or any combination thereof. A rangeof anion precursors for the core and the shell material may also beused, including, but not limited to, the element itself (oxidation state0), covalent compounds, or ionic compounds.

To examine the stability of the core CdSe nanocrystals, the UV-Visabsorption spectrum of these nanocrystals was examined after heating at240° C. for at least 30 minutes, without injecting any solution of shellmaterials. The UV-Vis absorption spectrum of the core CdSe nanocrystalsshowed no noticeable change as a result of this heat treatment. The PLQY of the CdSe core nanocrystals increased in the first 5-15 minutes ofthis heat treatment, (see: Talapin, D.; Rogach, A. L.; Kornowski, A.;Haase, M.; Weller, H. Nano Lett. 2001, 1, 207, which is incorporatedherein by reference in its entirety) and often decreased after theinitial heating period at 240° C. Higher temperatures, for example 260°C. and above, made it difficult to prevent Ostwald ripening of the coreand core/shell nanocrystals. Therefore, except where indicated, the datadescribed herein were obtained using shell growth reactions at abut 240°C.

FIG. 1 (middle plot) illustrates a spectral comparison of the CdSe/CdScore/shell nanocrystals with the same shell thickness, grown onto thesame batch of core nanocrystals under different temperatures. While notintending to be bound by theory, this comparison suggests that as thetemperature decreases, increasingly less of the cation precursor (inthis example, cadmium) and anion precursor (in this example, sulfur)injected into the solution grow onto the surface of the existingnanocrystals, but instead begin to form isolated CdS nanocrystalsthrough homogenous nucleation process. Thus, relative to the absorbanceat the first absorption peak, the absorbance in the wavelength rangebelow 500 nm for the low temperature reactions increased rapidly,indicating the formation of individual CdS nanocrystals.

In those cases where isolated CdS nanocrystals formed in the growthprocess, they could be separated readily from the CdSe/CdS core/shellnanocrystals, either by extraction or by precipitation. In one aspect,extraction purification could be carried out by extracting the pure CdSnanocrystals into the methanol layer from the hexanes/ODE layercontaining CdSe/CdS core/shell nanocrystals. Under UV irradiation, themethanol layer emitted blue, and the hexanes/ODE layer exhibited the PLof the core/shell nanocrystals. After purification, the PLE(photoluminescence excitation) spectra of the core/shell nanocrystalswere changed dramatically (FIG. 1, bottom plot). As shown in FIG. 1(bottom plot), the existence of isolated CdS particles in the solutionoften caused a significant drop of the PLE below 500 nm, which is likelydue to the absorption of isolated CdS nanocrystals in that opticalwindow.

The size, shape and size/shape distribution of the CdSe/CdS core/shellnanocrystals were controlled using SILAR. FIG. 2 exhibits four TEMimages of CdSe plain core nanocrystals and the corresponding core/shellnanocrystals with different shell thicknesses. The size increaseobserved by TEM correlated well with the numbers of CdS monolayersestimated from the injections. The nearly monodisperse core nanocrystalsformed well-developed, two-dimensional (2D) superlattices of thenanocrystals. Upon the growth of CdS shell, 2D superlattice was stillreadily observed if the shell thickness was one or two monolayers ofCdS. Core/shell nanocrystals with three or more monolayers of CdS weremostly ordered locally.

As indicated in FIG. 2, in one aspect of this invention, the shape ofthe core/shell nanocrystals appeared to depend on the shell thickness(FIG. 2). Nanocrystals with thin shells, for example one or twomonolayers, maintained the dotted shape of the core nanocrystals, andthus readily formed 2D superlattices. Nanocrystals with thicker shellswere observed to have somewhat elongated shapes, and 2D superlatticesdid not form if those nanocrystals were randomly oriented, whichoccurred at low particle concentrations. Thus, orientation of thoseelongated core/shell nanocrystals was achieved by depositing them from arelatively concentrated solution. Accordingly, as shown in FIG. 3, thismethod made it possible to obtain superlattice packing with thousands ofnanocrystals.

The appearance of almost all nanocrystals in the superlattice shown inFIG. 3 was dot-shaped. This observation reflects that the nanocrystalshad oriented themselves on the grids with their long axis perpendicularto the substrate. The selected area electron diffraction (SAED) patternsshown in FIG. 3 further revealed that the c-axis of the nanocrystals inthe superlattice was perpendicular to the substrate. These twoobservations suggested that the long axis of the core/shell nanocrystalswith relatively thick shells was the c-axis of the wrutzite structure,which was verified by High-resolution TEM measurements. As shown in FIG.4 (A and B), the shape of the 5-layer nanocrystals was significantlyelongated along the c-axis, and approached a petal-shape. The dimensionalong the c-axis of the top two nanocrystals in FIG. 4 is about 9 nm,while the dimension perpendicular to the c-axis is about 7 nm. A smallportion of the core/shell nanocrystals with 5-layer CdS were found tomore closely resemble a dot-shape (for example, see bottom two images inFIG. 4). Thus, overall, the distribution of the aspect ratio of thecore/shell nanocrystals with thick shells was not as narrow as that ofthe dimension of the short axis.

In one aspect of this invention, although 5-layer CdS is illustrated inthe Figures provided herein, the core/shell nanocrystals of thisinvention could be characterized by a thickness of the shell materialfrom 1 to about 15 monolayers. Thus, in this aspect of the invention,the thickness of any shell material could be 1, 2, 3, 4, 5, 6, 7, 8, 9,10, 11, 12, 13, 14, 15, or more monolayers.

For the CdSe/CdS core/shell nanocrystals illustrated in the Figures, theX-ray diffraction patterns of the core/shell nanocrystals shifted from awurtzite CdSe-like pattern to a wurtzite CdS-like pattern as the shellthickness increased upon successive CdS shell layering. For thecore/shell nanocrystals with five monolayers of CdS shell, thediffraction pattern was substantially the same as pure CdS nanocrystalsof the same size, as illustrated in FIG. 5. The crystal domain sizecalculated using the Sherrer Equation using the (110) peak is about 7nm, which is consistent with the TEM results discussed above. Thecomposition of a thick shell CdSe/CdS core/shell nanocrystal with fivemonolayers of CdS shell, is approximately 10:1 molar ratio of CdS:CdSe.The relatively low intensity and broad peak of the (103) peak incomparison to the (110) peaks in the diffraction pattern shown in FIG. 5are consistent with the existing stacking faults perpendicular to thec-axis observed by High Resolution TEM (HRTEM) as seen in FIG. 4, withone stacking fault per particle in average. (See: Murray, C. B.; Norris,D. J.; Bawendi, M. G. J. Am. Chem. Soc. 1993, 115, 8706-8715.)

In another aspect of this invention, for example, the UV-Vis and PLspectra of the core/shell nanocrystals of a typical reaction areillustrated in FIG. 6 (left panel). The sharp features of the absorptionspectra and the narrow PL peaks are consistent with the narrow sizedistribution of the core/shell nanocrystals shown in FIGS. 2 and 3.

In yet another aspect, for example, the PL spectrum of core/shellnanocrystals with thick shells often possessed a shoulder on the highenergy side, and the relative intensity of this shoulder increased bypurifying the nanocrystals by removing traces of the side products andinitial reactants, by the methods described in the Examples. Thephotoluminescence excitation, PLE, of the shoulder emission wasapproximately the same to that of the band edge PL (FIG. 6, rightpanel).

For the CdSe/CdS core/shell nanocrystals used to illustrate the SILARmethod, the photoluminescence quantum yields (PL QY) andphotoluminescence Full-Width-at-Half-Maximum peak width (PL FWHM) of thecore/shell nanocrystals vary as a function of shell thickness of thecore/shell nanocrystals, as follows. The PL QY of the core/shellnanocrystals was observed to increase as the shell thickness increased,as shown in FIG. 7. Additionally, the PL QY of the plain corenanocrystals prior to the shell growth also varied significantly even ifthe reactions were carried out with the same batch of the corenanocrystals as shown in FIG. 7. However, with the same batch ofnanocrystal cores, the peak positions, the PL QY, and the PL FWHM of thecore/shell nanocrystals generated by two parallel reactions were similar(FIG. 7).

In another aspect, growth reactions with TOPO-coated nanocrystal coresgenerated core/shell nanocrystals with significantly lower PL QY incomparison to the ones with amine-coated cores, although a systematicincrease of the PL QY was also observed upon the increase of the shellthickness (FIG. 7).

In one aspect, the photoluminescence Full-Width-at-Half-Maximum peakwidth (PL FWHM) of the core/shell nanocrystals maintained the originalvalue of the original cores within experimental error (FIG. 7, top).This observation is consistent with the good control of the sizedistribution of the core/shell nanocrystals described above. However,the PL spectra of the core/shell nanocrystals with thicker shells, forexample at least about five monolayers, often possessed a tailspectroscopic feature on the high energy side as described above.

Multigram-scale growth of the core/shell nanocrystals was also achievedusing the SILAR method disclosed herein. The quality of the resultingnanocrystals was similar to those obtained from small scale syntheses.The TEM images shown in FIGS. 2 and 3 were all recorded with thenanocrystals generated by a large scale synthesis which yielded about2.5 grams core/shell nanocrystals, and are representative of thoseobtained from small scale syntheses as well. In one aspect, even forthis large scale synthesis, dropwise addition of the precursor solutionswas not necessary to obtain high quality nanocrystals.

The Successive ionic layer adsorption and reaction (SILAR) method isalso applicable to many other semiconductor core/shell nanocrystals. Inone aspect, for example, high quality core/shell nanocrystals that canbe prepared by the SILAR method disclosed herein include, but are notlimited to, CdSe/CdS, CdSe/ZnSe, CdSe/ZnS, CdS/ZnS, CdTe/CdSe, CdTe/CdS,CdTe/ZnTe, CdTe/ZnSe, CdTe/ZnS, ZnSe/ZnS, ZnTe/CdS, ZnTe/ZnSe, InAs/InP,InAs/CdSe, InAs/CdS, InAs/ZnS, InP/CdS, InP/ZnS, InP/ZnSe, InAs/InP/CdS,CdS/CdSe, CdS/InP, and the like. Moreover, the optical properties andquality of core/shell nanocrystals prepared by the SILAR method areoften improved over those prepared by traditional methods. For example,InAs/InP core shell nanocrystals prepared using traditional approachesare reported to exhibit PL QY values lower than the plain core InAsnanocrystals (see: Cao, Y.; Banin, U. J. Am. Chem. Soc. 2000, 122,9692-9702). However, the core/shell nanocrystals with III-Vsemiconductor nanocrystals as the cores and synthesized using SILARtechnology encompassed by this invention, all possessed higher PL QY andbetter stability against photo-oxidation than the corresponding plaincore nanocrystals did. For example, using. ZnSe and ZnS as the shellmaterials, it was observed that the PL QY of highly luminescent CdSecore nanocrystals could be maintained at above 50% level. Thus,CdSe/ZnSe and CdSe/ZnS core/shell nanocrystals may be brighter emittingmaterials than the CdSe/CdS system. The last two core/shell systems,CdS/CdSe and CdS/InP, represent a new series of colloidal quantumstructures, quantum shells. Specific examples of the growth ofcore/shell nanocrystals using III-V semiconductor nanocrystals as thecores are illustrated in FIGS. 10 and 11.

In one aspect, darkening and brightening phenomena were observed in thepreparation of the core/shell nanocrystals of the present invention, asillustrated in FIG. 8. In the growth process, especially for thosereactions that occurred at relatively low temperatures, a darkeningprocess was observed immediately after each injection of the sulfurstock solution. When this darkening was observed, a rapid increase ofthe shell thickness, as indicated by a rapid red shift of the absorptionpeak, was typically observed. In the case of the CdSe/CdS core/shellnanocrystals, annealing sometimes resulted in a brightening phenomenon,thus-improved the PL QY of these nanocrystals, accompanied by a smallred-shift of the emission peak (see FIG. 8).

The CdSe/CdS core/shell nanocrystals described herein could bebrightened by laser photo-irradiation when oxygen was present in thesolution. Brightening was always accompanied by a noticeable blue shiftof the absorption/PL spectrum, suggesting the size of the semiconductornanocrystals decreased. While not intending to be bound by this theory,these results imply that the photo-induced brightening phenomenonobserved here is due to the photo-oxidation of the shell material. Therate of this photo-oxidation of core/shell nanocrystals was stronglydependent on the environment, although it occurred in a much slower ratein comparison to the corresponding plain core nanocrystals. In water orpolar solvents, photo-oxidation of hydrophilic thiol-coated CdSe/shellnanocrystals occurred easily, as illustrated in FIG. 9. In non-polarsolvents, the oxidation of the ODA-coated core/shell nanocrystalsrequired from hours to days to show a noticeable change. When thecore/shell nanocrystals were embedded in thin polymer film, nophoto-oxidation was observed in air with intense laser radiation for atleast 1-2 hours. It was observed that those core/shell nanocrystalsembedded in thin polymer film did not blink as frequently as thecorresponding core nanocrystals, with most nanocrystals existing at “on”state for most of the time.

In another aspect of this invention, a brightening phenomenon wasobserved when the nanocrystals were purified using either the extractionor precipitation method. The PL QY of the core/shell nanocrystalsincreased by the removal of the side products and unreacted precursors(see FIG. 8), although the emission peak retained the same positionafter purification. The shoulder at the high energy side of the PLspectrum of the core/shell nanocrystals with a thick shell often becamemore obvious after purification (FIG. 8).

The processability of the core/shell nanocrystals was superior to thatof the corresponding core nanocrystals, as illustrated in FIG. 9. Thus,the PL QY of core/shell nanocrystals increased by purificationprocedures through either precipitation or extraction. However, thehighly luminescent CdSe core nanocrystals became barely emitting after aparallel precipitation procedure. In one aspect of this invention, inthe case of the plain core nanocrystals, in order to maintain areasonable PL brightness, the purification processing is typicallystopped when there is still a significant amount of free amine in thesolution along with the plain core nanocrystals.

The presence of free amine in the solution is know to adversely affectthe PL emission efficiency of plain core nanocrystals when deposited asthin films for LEDs, lasers, and the like, as the films tend to becomeopaque. In contrast, purified core/shell nanocrystals formed opticallyclear thin films using the same procedures. Furthermore, even though thecore/shell nanocrystals in solution were often less bright than theplain core nanocrystals dissolved in the same solvent, they possessedhigher PL QY than the plain core nanocrystals did when both types ofnanocrystals were in the form of solid films, as illustrated in FIG. 9.

In one aspect of this invention, one typical method to converthydrophobic semiconductor nanocrystals into water soluble semiconductornanocrystals, for bio-medical applications and the like, is to replacethe original hydrophilic ligands by hydrophilic thiol ligands. (See, forexample: Bruchez, M.; Moronne, M.; Gin, P.; Weiss, S.; Alivisatos, A. P.Science 1998, 281, 2013-2016; Chan, W. C. W.; Nie, S. M. Science 1998,281, 2016-2018; Mattoussi, H.; Mauro, J. M.; Goldman, E. R.; Anderson,G. P.; Sundar, V. C.; Mikulec, F. V.; Bawendi, M. G. J. Am. Chem. Soc.2000, 122, 12142.) Highly emitting, plain core nanocrystals became notemitting at all after the amine ligands were replaced by hydrophilicthiols, as shown in FIG. 9. In contrast, the PL of core/shellnanocrystals remained to some extent after the same treatment withhydrophilic thiols. Furthermore, the PL brightness of the thiol-coatedcore/shell CdSe/CdS nanocrystals can be recovered by the controlledphoto-chemical (as illustrated in FIG. 8) or chemical oxidations.However, the same oxidation treatments usually decomposed the plain coreCdSe nanocrystals rapidly, without any noticeable recovery of thephotoluminescence. These results suggest that the CdSe/CdS core/shellnanocrystals are substantially more stable than the corresponding plaincore nanocrystals under certain harsh chemical and thermal processingconditions.

The epitaxial growth feature of the resulting nanocrystals could bedetermined using the criteria established previously, as described inPeng, X.; Schlamp, M. C.; Kadavanich, A. V.; Alivisatos, A. P. J. Am.Chem. Soc. 1997, 119, 7019-7029, which is incorporated herein in itsentirety by reference. The immediate and significant red-shift of theabsorption spectra upon the growth of each monolayer of CdS (illustratedin FIGS. 1 and 6) revealed that the resulting nanocrystals arecore/shell structures, instead of alloy particles. The TEM and XRDmeasurements (see FIGS. 4 and 5) indicated that the core/shellnanocrystals are single crystals. The lattice fringes including thestacking faults (FIG. 4) of each nanocrystal extended completely acrosseach nanocrystal, which is a hallmark of epitaxial growth.

Thus, in one aspect, the present invention provides core/shellnanocrystals, and a composition comprising core/shell nanocrystals,wherein:

the nanocrystals comprise a core material and a shell materialovercoating the core material, each of which is independently selectedfrom a II/VI compound or a III/V compound,

the band gap of the core material is less than the band gap of the shellmaterial; and

the thickness of the shell material is from 1 to about 15 monolayers.

Thus, in this aspect of the invention, the thickness of the shellmaterial could be 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, ormore monolayers, and still maintain the monodispersity of the core/shellnanocrystals.

In another aspect, the core/shell nanocrystals of this invention mayexhibit a type-I band offset or a type-II band offset. Further, and inanother aspect, examples of the core/shell nanocrystals of thisinvention include, but are not limited to, CdSe/CdS, CdSe/ZnSe,CdSe/ZnS, CdS/ZnS, CdTe/CdSe, CdTe/CdS, CdTe/ZnTe, CdTe/ZnSe, CdTe/ZnS,ZnSe/ZnS, ZnTe/CdS, ZnTe/ZnSe, InAs/InP, InAs/CdSe, InAs/CdS, InAs/ZnS,InP/CdS, InP/ZnS, InP/ZnSe, InAs/InP/CdS, CdS/CdSe, CdS/InP, or amixture thereof.

In still another aspect, the as-prepared core/shell nanocrystals canexhibit a photoluminescence quantum yield (PL QY) up to about 40%. Inanother aspect, the core/shell nanocrystals can photoluminesce at awavelength from about 400 to about 1000 nm. In yet another aspect, thecore/shell nanocrystals can exhibit a photoluminescence emission linecharacterized by a FWHM of about 60 nm or less, about 55 nm or less,about 50 nm or less, about 45 nm or less, about 40 nm or less, about 35nm or less, about 30 nm or less, about 28 nm or less, or about 25 nm orless.

In another aspect, the core/shell nanocrystals of this invention aremonodisperse. In this aspect, the core/shell nanocrystals can becharacterized by a size distribution having a standard deviation nogreater than about 15% of a mean diameter of the population ofcore/shell nanocrystals, no greater than about 12% of a mean diameter ofthe population of core/shell nanocrystals, no greater than about 10% ofa mean diameter of the population of core/shell nanocrystals, no greaterthan about 7% of a mean diameter of the population of core/shellnanocrystals, or no greater than about 5% of a mean diameter of thepopulation of core/shell nanocrystals.

In another aspect, this invention provides devices comprising thecore/shell nanocrystals and compositions of this invention, including,but not limited to, light-emitting diodes, biological labeling agents,photoelectric devices, solar cells, lasers, and the like.

In yet another aspect, this invention provides a population ofnanocrystals comprising a plurality of nanocrystals, wherein:

each nanocrystal comprises a core material and the shell materialovercoating the core material, each of which is independently selectedfrom a II/VI compound or a III/V compound,

wherein the band gap of the core material is less than the band gap ofthe shell material;

wherein the population of nanocrystals is substantially monodisperse;and

wherein the plurality of nanocrystals exhibit a photoluminescencequantum yield (PL QY) of greater than or equal to about 20%.

In this aspect, for example, the core material can be selected fromCdSe, CdS, InAs, or InP; the shell material can be selected from CdS,CdSe, ZnSe, ZnS, or InP; and the shell material is different from thecore material.

Also in this aspect, the plurality of nanocrystals can exhibit aphotoluminescence quantum yield (PL QY) of greater than or equal toabout 5%, greater than or equal to about 10%, greater than or equal toabout 20%, greater than or equal to about 30%, greater than or equal toabout 40%, greater than or equal to about 50%, greater than or equal toabout 60%, or greater than or equal to about 70%.

In yet another aspect, this invention provides a population ofcore/shell nanocrystals, wherein:

the core material is selected from CdSe, CdS, CdTe, ZnSe, ZnS, ZnTe,HgSe, HgS, HgTe, ZnO, CdO, GaAs, InAs, GaP, or InP;

the shell material is selected from CdSe, CdS, CdTe, ZnSe, ZnS, ZnTe,HgSe, HgS, HgTe, ZnO, CdO, GaAs, InAs, GaP, or InP; and

the shell material is different from the core material.

In another aspect, the plurality of nanocrystals of this invention aresubstantially monodisperse. In this aspect, the plurality ofnanocrystals can be characterized by a size distribution having astandard deviation no greater than about 15% of a mean diameter of thepopulation, no greater than about 12% of a mean diameter of thepopulation, no greater than about 10% of a mean diameter of thepopulation, no greater than about 7% of a mean diameter of thepopulation, or no greater than about 5% of a mean diameter of thepopulation.

In still another aspect, the plurality of nanocrystals can exhibit aphotoluminescence quantum yield (PL QY) from about 20% to about 40%. Inanother aspect, the plurality of nanocrystals can photoluminesce at awavelength from about 400 to about 1000 nm. In yet another aspect, theplurality of nanocrystals can exhibit a photoluminescence emission linecharacterized by a FWHM of about 60 nm or less, about 55 nm or less,about 50 nm or less, about 45 nm or less, about 40 nm or less, about 35nm or less, about 30 nm or less, about 28 nm or less, or about 25 nm orless.

Theoretical Considerations

While not intending to be bound by the following theory, it is believedthat the properties of the CdSe/CdS core/shell system produced by SILARtechnology can be explained as follows. The photo-generated excitons inthe CdSe/CdS core/shell system are delocalized in both core and shellmaterials, with holes mostly confined in the core and electronsdelocalized in both core and shell. (See: Peng, X.; Schlamp, M. C.;Kadavanich, A. V.; Alivisatos, A. P. J. Am. Chem. Soc. 1997, 119,7019-7029.) The delocalization of the excitons fosters theshell-thickness dependent optical properties of CdSe/CdS core/shellsystem, which provides us convenient probes to study the growth ofcore/shell nanocrystals. However, the poor overlap of the wavefunctionsof the photo-generated hole and electron makes CdSe/CdS core/shellnanocrystals not emit as well as CdSe/ZnS or CdSe/ZnSe core/shellnanocrystals, as observed. In one aspect, for example, the PL QY of theas-prepared CdSe/ZnSe and CdSe/ZnS nanocrystals synthesized using SILARcould reach above 50%. In molecular beam epitaxy-grown (MBE-grown)quantum wells of CdSe—CdS system, the poor overlap of the hole andelectron of excitons was found to induce so-called spatially indirect PL(photoluminescence), with the hole mostly in the CdSe and the electronmostly in CdS layer. (See, for example: Langbein, W.; Hetterich, M.;Gruen, M.; Klingshirn, C.; Kalt, H. Appl. Phys. Lett. 1994, 65, 2466-68,which is incorporated herein by reference in its entirety.) Again, whilenot intending to be bound by theory, the high energy shoulder observedfor CdSe/CdS core/shell nanocrystals with a thick shell is likely due tosuch spatially indirect PL. As shown in FIG. 6 (right), although theshoulder was about 35 nm separated from the emission peak, the PLEspectra of the two emission features were quite similar. Thisobservation likely indicates that the shoulder in the PL spectrum shouldnot be due to either the emission from isolated CdS nanocrystals or theemission of another set of sizes. As illustrated in FIG. 8, purificationoften increased the relative PL intensity of the shoulder emission incomparison to that of the peak itself. This characteristic suggests thatthe shoulder emission may possess more of a surface feature, which isconsistent with the delocalization of the hole into the CdS shell layerrequired for the spatially indirect emission. This interesting PLproperty of CdSe/CdS system was likely not observed previously becauseof the less controlled growth, which made it difficult to grow thick CdSshells.

The photoluminescence (PL) properties of highly crystallinesemiconductor nanocrystals are strongly dependent on the surface of thenanocrystals. This dependence also applies to the CdSe/CdS core/shellnanocrystals due to the significant delocalization of the excitons inthis system as discussed herein. Again, while not intending to be boundby theory, the brightening and darkening phenomena shown in FIG. 8 areall probably related to the improvement of the surface structure andsurface environment of the nanocrystals. The darkening phenomenonobserved right after the introduction of the sulfur precursors isconsistent with the accompanied rapid growth of the core/shellnanocrystals since a rapid growth often causes the disorder of thesurface of nanocrystals. (See, for example: Qu, L; Peng, X. J. Am. Chem.Soc. 2002, 124, 2049-2055; Talapin, D. V.; Rogach, A. L.; Shevchenko, E.V.; Kornowski, A.; Haase, M.; Weller, H. J. Am. Chem. Soc. 2002, 124,5782-5790, which is incorporated herein by reference in its entirety.) Asimilar explanation can be applied to the brightening observed uponannealing. The improvement of emission properties of the core/shellnanocrystals by purification is possibly related to the removal of somecritical impurities in the growth solution which quenches the PL of thecore/shell nanocrystals.

In one aspect, photo-oxidation and chemical oxidation was observed toimprove the PL QY of CdSe/CdS core/shell nanocrystals significantlyalthough it typically does not improve the PL brightness of plain coreCdSe nanocrystals. While not intending to be bound by theory, onepossible reason is that the bond length of Cd—O bond is much smallerthan that of the Cd—Se bond. As a result, the CdO formed by oxidationmight not stay on the surface of the remaining CdSe nanocrystal cores.In comparison, the Cd—S bond length is about 5-6% shorter than that ofthe Cd—Se bond. Therefore, a thin layer of CdO might adhere to thesurface of the remaining core/shell nanocrystals, resulting in aCdSe/CdS/CdO complex structure. Thus, it is possible that the extra CdOshell could further provide electronic passivation for the CdSe cores.Thus, if the above mechanism is correct, oxidation of core/shellnanocrystals with ZnS or ZnSe shell should also improve their PL QY.However, since the absorption and PL peak positions of CdSe/ZnS andCdSe/ZnSe core/shell nanocrystals are nearly insensitive to the shellthickness of the nanocrystals, it is difficult to distinguish theinfluence of photo-oxidation from other factors, such asphoto-annealing.

This invention demonstrates (see, for example FIG. 9) that ODA ligandsare bound tightly on the surface of the CdSe/CdS core/shell nanocrystalsdescribed herein. However, this is not the case for either thecorresponding CdSe core nanocrystals (Qu, L.; Peng, X. J. Am. Chem. Soc.2002, 124, 2049-2055, which is incorporated herein by reference in itsentirety.) or the CdSe/CdS core/shell nanocrystals synthesized atrelatively low temperatures (Peng, X.; Schlamp, M. C.; Kadavanich, A.V.; Alivisatos, A. P. J. Am. Chem. Soc. 1997, 119, 7019-7029, which isincorporated herein by reference in its entirety.). While not intendingto be bound by this theory, the enhanced bonding between ODA and theCdSe/CdS core/shell nanocrystals described herein in comparison to theCdSe core nanocrystals can be explained by the relatively strong bondingof the CdS surface with electron donating species. The differencebetween the CdSe/CdS core/shell nanocrystals synthesized at differenttemperatures implies that higher temperatures promote the surfacepassivation with ODA, either through strong bonding of the ODA ligands,high surface coverage, or both.

Quantum Shells, Quantum Wells, Doped Nanocrystals, 0D-2D Nanocrystals,and Other Complex Structured Nanocrystals

In another aspect, the present invention provides new types of colloidalmaterials including, but not limited to, quantum shells, quantum wells,doped nanocrystals, 0D-2D nanocrystals, and other complex structurednanocrystals.

As compared to core/shell nanocrystals with a small bandgap core and alarge bandgap shell, with a type-I band offset, as described above, whenthe bandgap structure is reversed and a large bandgap is present for thecore and a small bandgap is present for the shell material, theresulting core/shell semiconductor nanocrystals may approach thebehavior of a two-dimensional (2D) system, for which the photo-generatedholes and electrons are quantum confined inside the shell material only.While not intending to be bound by theory, for a 2D system, the diameterof the exciton is generally smaller than the perimeter of the shell inorder to have the quantum confinement occur radially only. In thisapplication, an “exciton” refers to the weakly bound electron-hole pairgenerated by photo-excitation. (See, for example: A. R. Kortan, R. Hull,R. L. Opila, M. G. Bawendi, M. L. Steigerwald, P. J. Carroll, L. E.Brus, J. Am. Chem. Soc. 112 (1990) 1327; A. Mews, A. Eychmueller, M.Giersig, D. Schooss, H. Weller, J. Phys. Chem. 98 (1994) 934; Y. Tian,T. Newton, N. A. Kotov, D. M. Guldi, J. H. Fendler, J. Phys. Chem. 100(1996) 8927; R. B. Little, M. A. El-Sayed, G. W. Bryant, S. Burke, J.Chem. Phys. 114 (2001) 1813; each of which is incorporated by referenceherein, in its entirety.)

Thus, in one aspect of this invention, quantum shells represent a newclass of colloidal quantum structures grown by SILAR. In this aspect,for example, one difference between quantum shells and regularcore/shells is the energy band gap offset between the core and shellmaterials. Thus, for quantum shells, the band gap of the shell issubstantially smaller than that of the core material. As a result, thephoto-generated excitons will be localized in the shell for quantumshells. While not intending to be bound by theory, if the diameter ofthe exciton of the shell material is smaller than the perimeter of theshell, the radial quantum confinement is so much stronger than the othertwo dimensions, this system behaves like a 2D system.

The number of possible quantum shell systems is very large. In oneaspect of the invention, two specific examples of quantum shells,CdS/CdSe and CdS/InP, are described in detail herein. However, thepresent invention provides any colloidal core/shell semiconductornanocrystal system with a narrow bandgap material as the shell and awide bandgap material, including an insulator, as the core. Thus, thepresent invention comprises new compositions comprising quantum shells,new methods to prepare quantum shells, and new devices comprisingquantum shells.

Traditional core/shell nanocrystals typically comprise a small bandgapcore and a large bandgap shell. While not intending to be bound bytheory, when the bandgap structure is reversed, a large bandgap for thecore and a small bandgap for the shell material, the resultingcore/shell semiconductor nanocrystals may behave like a two-dimensional(2D) system, for which the photo-generated holes and electrons arequantum confined inside the shell material only. Again, while notintending to be bound by theory, if the diameter of the photo-generatedexcitons is smaller than the perimeter of the shell, the quantumconfinement only occurs radially. Thus, the present invention comprisesthese “reversed” bandgap structures comprising a large bandgap core anda small bandgap shell material, that results in quantum shellscore/shell semiconductor nanocrystals.

The unique optical properties of quantum shells make them excellentemitters for many applications. The global Stocks shifts of quantumshells (as illustrated in FIGS. 12 and 13) are significantly larger thanthe corresponding quantum dots, especially compared to medium and largesized quantum dots in the case of CdSe. The molar extinction coefficientat the emission peak wavelength of quantum shells, as illustrated inFIG. 14, is very small in comparison to the quantum dots with the sameemission peak position. Therefore, it was discovered that quantum shellsare expected to be ideal emitters when the re-absorption ofphotoluminescence or electroluminescence hinders the performance of aparticular device. For example, performance may be affected byre-absorption and Foster energy transfer of photoluminescence orelectroluminescence in solid state lasers, LEDs, in bio-medical labelingdevices using semiconductor nanocrystals, especially if multiplelabeling is employed.

FIG. 15 provides the PLE spectra of two samples of CdS/CdSe quantumshells with a shell thickness of five monolayers applied to differentsize cores. The similarity of the electronic energy states of these twosystems suggests a 1D confinement system.

In another aspect, more complex structured nanocrystals can also begrown using SILAR technique. For example, the growth of severalmonolayers of CdS onto the CdS/CdSe quantum shells was found to form anew class of nanocrystals, colloidal quantum wells. The resultingnanocrystals structurally and electronically resemble theMolecular-Beam-Epitaxy (MBE) quantum wells. The emission properties ofquantum shells could be improved through inorganic passivation byadditional epitaxial growth of a high bandgap semiconductor on thesurface of the quantum shells in a one-pot approach. FIG. 17 shows theoptical spectra before and after the epitaxial growth of four monolayersof CdS onto a CdSe quantum shell sample. As expected, the PL QYincreased significantly after the inorganic overcoating and the PL peakposition shifted to red. For inorganically-passivating quantum shellswith CdS, it was seen that using fatty acids, including but not limitedto oleic acid (OA) as ligands, yielded nanocrystals with higher quantumyields than did ones using amines. When fatty acids were used as theligands, the CdSe quantum shells before the inorganic passivation didnot emit well but did grow the desired monolayers on the CdS templates.This observation is consistent with the observation that fatty acids arebetter passivation ligands for CdS surfaces and amines are betterchoices for CdSe surface as discussed above.

In another aspect of this invention, and different from quantum shells,the present invention provides quantum wells, including, but not limitedto, CdS/CdSe/CdS quantum wells. It was observed that those quantum wellswith a single monolayer of CdSe emit very well. This observation islikely because of the inorganic passivation provided by the outer CdSshell. Typically, the PL QY increased for the quantum wells as thethickness of the outer CdS shell increased. The PL QY reached a plateauafter about five to six monolayers of outer (second) shell CdS wasdeposited onto the quantum shells. The growth of quantum wells wasperformed in a single-pot fashion, which means that the quantum shellswere not isolated before the deposition of the outer shell material ontothe core/shell nanocrystal.

In another aspect of this invention, the SILAR technique was used forthe formation of very complex structured nanocrystals. In one aspect,for example, CdSe/ZnS/CdSe nanocrystals represent one class of suchcomplex nanocrystals. In this aspect, for example, the CdSe core behavesas 0D quantum system, and the CdSe outer shell constitutes a typicalquantum shell similar to the ones described herein. The ZnS first shellacts as the energy barrier for the core and the quantum shell.Consequently, the core (0D) and the quantum shell (2D) cannotcommunicate electronically and will respond to photo-excitation orelectronic-excitation independently. In this aspect, when thephotoluminescence of the core is not or cannot be absorbed by thequantum shell, photoluminescence from both core and quantum shell can beobserved, as illustrated in FIG. 18.

In another aspect of this invention, doped nanocrystals with a precisepositioning of the dopants along the radial direction can be synthesizedby the SILAR method. Such nanocrystals can be referred to as“radially-doped” or simply “doped” nanocrystals. For example, during thegrowth of a given monolayer, doping can be achieved by adding thedopants, either cationic or anionic, into the corresponding shellprecursor solution.

In another aspect, this invention provides a composition comprisingcore/multiple shell nanocrystals which are optionally doped at the coreand optionally doped at any shell. In this aspect, the present inventionprovides a composition comprising radially-doped, core/multiple shellnanocrystals, comprising:

1) a core material having the formula M¹ _(a-c)M² _(c)E¹ _(b), wherein:

-   -   a) M¹ is selected from a metal, E¹ is selected from a non-metal,        and a and b are dictated by the stoichiometry of the compound M¹        _(a)E¹ _(b);    -   b) M² is selected from a transition metal, a rare earth metal,        or a mixture thereof; and M² is different than M¹; and    -   c) 0≦c<a;

2) an optional first shell material overcoating the core material,having the formula M³ _(d-f)M⁴ _(f)E³ _(e), wherein:

-   -   a) M³ is selected from a metal, E³ is selected from a non-metal,        and d and e are dictated by the stoichiometry of the compound M³        _(d)E³ _(e);    -   b) M⁴ is selected from a transition metal, a rare earth metal,        or a mixture thereof; and M⁴ is different than M³; and    -   c) 0≦f<d;

3) an optional second shell material overcoating the optional firstshell material, having the formula M⁵ _(g-i)M⁶ _(i)E⁵ _(h), wherein:

-   -   a) M⁵ is selected from a metal, E⁵ is selected from a non-metal,        and g and h are dictated by the stoichiometry of the compound M⁵        _(g)E⁵ _(h);    -   b) M⁶ is selected from a transition metal, a rare earth metal,        or a mixture thereof; and M⁶ is different than M⁵; and    -   c) 0≦i<g;

4) an optional third shell material overcoating the optional secondshell material, having the formula M⁷ _(j-l)M⁸ _(l)E⁷ _(k), wherein:

-   -   a) M⁷ is selected from a metal, E⁷ is selected from a non-metal,        and j and k are dictated by the stoichiometry of the compound M⁷        _(j)E⁷ _(k);    -   b) M⁸ is selected from a transition metal, a rare earth metal,        or a mixture thereof; and M⁸ is different than M⁷; and    -   c) 0≦l<j; and

5) an optional fourth shell material overcoating the optional thirdshell material, having the formula M⁹ _(m-o)M¹⁰ _(o)E⁹ _(n), wherein:

-   -   a) M⁹ is selected from a metal, E⁹ is selected from a non-metal,        and m and n are dictated by the stoichiometry of the compound M⁹        _(m)E⁹ _(n);    -   b) M¹⁰ is selected from a transition metal, a rare earth metal,        or a mixture thereof; and M¹⁰ is different than M⁹; and    -   c) 0≦o<m.

In another aspect, this invention provides a composition comprisingradially-doped, core/multiple shell nanocrystals, wherein M¹ _(a)E¹_(b), M³ _(d)E³ _(e), M⁵ _(g)E⁵ _(h), M⁷ _(j)E⁷ _(k), and M⁹ _(m)E⁹ _(n)are independently selected from a compound or a III/V compound.

In yet another aspect, this invention provides a composition comprisingradially-doped, core/multiple shell nanocrystals, wherein the thicknessof the first shell material, the second shell material, the third shell,and the fourth shell material are independently varied between 1 andabout 15 monolayers.

In another aspect, doped, core/multiple shell nanocrystals arecharacterized by a band gap of any shell material is less than the bandgap of the both adjacent core or shell materials, or greater than theband gap of the both adjacent core or shell materials.

Still another aspect of this invention are core/multiple shellnanocrystals having the formula M¹ _(a-c)M² _(c)E¹ _(b)/M³ _(d-f)M⁴_(f)E³ _(e)/M⁵ _(g-i)M⁶ _(i)E⁵ _(h)/M⁷ _(j-l)M⁸ _(l)E⁷ _(k)/M⁹ _(m-o)M¹⁰_(o)E⁹ _(n), wherein:

a) i) M¹, M³, M⁵, M⁷, and M⁹ are independently selected from Zn, Cd, orHg, and E¹, E³, E⁵, E⁷, and E⁹ are independently selected from O, S. Se,or Te; or

-   -   ii) M¹, M³, M⁵, M⁷, and M⁹ are independently selected from Ga        and In, and E¹, E³, E⁵, E⁷, and E⁹ are independently selected        from N, P and As; and

b) M², M⁴, M⁶, M⁸, and M¹⁰ are independently selected from Mn, Fe, Co,Ni, Pd, Pt, Cu, Al, Ag, or Au, or a rare earth metal. In another aspect,the doped core/multiple shell nanocrystals of this invention have theformula M¹ _(a-c)M² _(c)E¹ _(b)/M³ _(d-f)M⁴ _(f)E³ _(e)/M⁵ _(g-i)M⁶_(i)E⁵ _(h)/M⁷ _(j-l)M⁸ _(l)E⁷ _(k); in another aspect, have the formulaM¹ _(a-c)M² _(c)E¹ _(b)M³ _(d-f)M⁴ _(f)E³ _(e)/M⁵ _(g-i)M⁶ _(i)E⁵ _(h);in yet another aspect, have the formula M¹ _(a-c)M² _(c)E¹ _(b)/M³_(d-f)M⁴ _(f)E³ _(e); and in still another aspect, have the formula M¹_(a-c)M² _(c)E¹ _(b).

In yet another aspect, the present invention provides dopedcore/multiple shell nanocrystal having the formula M¹ _(a-c)M² _(c)E¹_(b)/M³ _(d-f)M⁴ _(f)E³ _(e)/M⁵ _(g-i)M⁶ _(i)E⁵ _(h)/M⁷ _(j-l)M⁸ _(l)E⁷_(k)/M⁹ _(m-o)M¹⁰ _(o)E⁹ _(n), wherein M³ _(d-f)M⁴ _(f)E³ _(e), M⁵_(g-i)M⁶ _(i)E⁵ _(h), M⁷ _(j-l)M⁸ _(l)E⁷ _(k), and M⁹ _(m-o)M¹⁰ _(o)E⁹_(n) constitute optional shell layers that are optionally doped, andwherein M¹ _(a)E¹ _(b), M³ _(d)E³ _(e), M⁵ _(g)E⁵ _(h), M⁷ _(j)E⁷ _(k),and M⁹ _(m)E⁹ _(n) are independently selected from CdSe, CdS, CdTe, ZnS,ZnSe, ZnTe, HgS, HgSe, HgTe, InAs, InP, GaAs, GaP, ZnO, CdO, HgO, In₂O₃,TiO₂, or a rare earth oxide.

In another aspect, the present invention provides doped core/multipleshell nanocrystals, wherein:

a) the nanocrystals comprise a core material, a first shell material,and a second material; and have the formula M¹ _(a-c)M² _(c)E¹ _(b)/M³_(d-f)M⁴ _(f)E³ _(e)/M⁵ _(g-i)M⁶ _(i)E⁵ _(h); and

b) the nanocrystals comprise ZnSe/Zn_(d-f)M⁴ _(f)Se/ZnSe,ZnSe/Zn_(d-f)M⁴ _(f)Se/ZnS, ZnO/Zn_(d-f)M⁴ _(f)O/ZnO, ZnO/Zn_(d-f)M⁴_(f)O/ZnS, TiO₂/Ti_(d-f)M⁴ _(f)O₂/TiO₂, and wherein M⁴ is selected fromMn, Fe, Co, Ni, Pd, Pt, Cu, Al, Ag, or Au, or a rare earth metal.

In still another aspect, this invention provides a method for preparingradially-doped core/multiple shell nanocrystals comprising a doped corematerial having the formula M¹ _(a-c)M² _(c)E¹ _(b), an optional dopedfirst shell material having the formula M³ _(d-f)M⁴ _(f)E³ _(e) andovercoating the core material, an optional doped second shell materialhaving the formula M⁵ _(g-i)M⁶ _(i)E⁵ _(h) and overcoating the firstcore material, an optional doped third shell material having the formulaM⁷ _(j-l)M⁸ _(l)E⁷ _(k) and overcoating the second core material, and anoptional doped fourth shell material having the formula M⁹ _(m-o)M¹⁰_(o)E⁹ _(n) and overcoating the third core material, comprising:

a) providing a solution of core nanocrystals of the formula M¹ _(a)E¹_(b), wherein M¹ is selected from a metal, E¹ is selected from anon-metal, and a and b are dictated by the stoichiometry of thecompound;

b) optionally forming at least one monolayer of a doped core material ofthe formula M¹ _(a-c)M² _(c)E¹ _(b) by contacting the nanocrystals, inan alternating manner, with a first cation precursor solution in anamount effective to form a monolayer of a first cation M¹, optionallydoped with a second cation M², and a first anion (E¹) precursor solutionin an amount effective to form a monolayer of the first anion;

-   -   wherein the first cation precursor solution comprises a first        cation (M¹) precursor and an optional second cation (M²)        precursor; and    -   wherein M² is selected from a transition metal, a rare earth        metal, or a mixture thereof; M² is different than M¹; and 0≦c<a;

c) optionally forming at least one monolayer of a doped first shellmaterial of the formula M³ _(d-f)M⁴ _(f)E³ _(e) by contacting thenanocrystals, in an alternating manner, with a second cation precursorsolution in an amount effective to form a monolayer of a third cationM³, optionally doped with a fourth cation M⁴, and a second anion (E³)precursor solution in an amount effective to form a monolayer of thesecond anion;

-   -   wherein the second cation precursor solution comprises a third        cation (M³) precursor and an optional fourth cation (M⁴)        precursor; and    -   wherein M³ is selected from a metal, E³ is selected from a        non-metal, and d and e are dictated by the stoichiometry of the        compound M³ _(d)E³ _(e);    -   wherein M⁴ is independently selected from a transition metal, a        rare earth metal, or a mixture thereof; M⁴ is different than M³;        and 0≦f<d;

d) optionally repeating step c to form optional doped shells M⁵ _(g-i)M⁶_(i)E⁵ _(h), M⁷ _(j-l)M⁸ _(l)E⁷ _(k), and M⁹ _(m-o)M¹⁰ _(o)E⁹ _(n),wherein M⁵, M⁷, and M⁹ are independently selected from a metal; M⁶, M⁸,and M¹⁰ are independently selected from a transition metal, a rare earthmetal, or a mixture thereof; E⁵, E⁷, and E⁹ are independently selectedfrom a non-metal; g and h are dictated by the stoichiometry of thecompound M⁵ _(g)E⁵ _(h); j and k are dictated by the stoichiometry ofthe compound M⁷ _(j)E⁷ _(k), m and n are dictated by the stoichiometryof the compound M⁹ _(m)E⁹ _(n); 0≦i<g; 0≦l<j; and 0≦o<m.

In this aspect, for example, this method can be used for preparing dopedcore/multiple shell nanocrystals wherein:

a) i) M¹, M³, M⁵, M⁷, and M⁹ are independently selected from Zn, Cd, orHg, and E¹, E³, E⁵, E⁷, and E⁹ are independently selected from O, S. Se,or Te; or

-   -   ii) M¹, M³, M⁵, M⁷, and M⁹ are independently selected from Ga        and In, and E¹, E³, E⁵, E⁷, and E⁹ are independently selected        from N, P and As; and

b) M², M⁴, M⁶, M⁸, and M¹⁰ are independently selected from Mn, Fe, Co,Ni, Pd, Pt, Cu, Al, Ag, or Au, or a rare earth metal.

In another aspect, for example, this method can be used for preparingdoped core/multiple shell nanocrystals wherein M¹ _(a)E¹ _(b), M³ _(d)E³_(e), M⁵ _(g)E⁵ _(h), M⁷ _(j)E⁷ _(k), and M⁹ _(m)E⁹ _(n) areindependently selected from CdSe, CdS, CdTe, ZnS, ZnSe, ZnTe, HgS, HgSe,HgTe, InAs, InP, GaAs, GaP, ZnO, CdO, HgO, In₂O₃, TiO₂, or a rare earthoxide.

In another aspect, for example, this method can be used for preparingdoped core/multiple shell nanocrystals wherein M¹ _(a)E¹ _(b), M³ _(d)E³_(e), M⁵ _(g)E⁵ _(h), M⁷ _(j)E⁷ _(k), and M⁹ _(m)E⁹ _(n) areindependently selected from a II/VI compound or a III/V compound.

In still another aspect, the thickness of the first shell material, thesecond shell material, the third shell, and the fourth shell materialare independently varied between 1 and about 15 monolayers.

In yet another aspect, the band gap of any shell material is less thanthe band gap of the both adjacent core or shell materials, or greaterthan the band gap of the both adjacent core or shell materials.

In another aspect, for example, this method can be used for preparingdoped core/multiple shell nanocrystals wherein:

a) i) M¹, M³, M⁵, M⁷, and M⁹ are independently selected from Zn, Cd, orHg, and E¹, E³, E⁵, E⁷, and E⁹ are independently selected from O, S. Se,or Te; or

-   -   ii) M¹, M³, M⁵, M⁷, and M⁹ are independently selected from Ga        and In, and E¹, E³, E⁵, E⁷, and E⁹ are independently selected        from N, P and As; and

b) M², M⁴, M⁶, M⁸, and M¹⁰ are independently selected from Mn, Fe, Co,Ni, Pd, Pt, Cu, Al, Ag, or Au, or a rare earth metal.

In yet another aspect, for example, this method can be used forpreparing doped core/multiple shell nanocrystals wherein M¹ _(a)E¹ _(b),M³ _(d)E³ _(e), M⁵ _(g)E⁵ _(h), M⁷ _(j)E⁷ _(k), and M⁹ _(m)E⁹ _(n) areindependently selected from CdSe, CdS, CdTe, ZnS, ZnSe, ZnTe, HgS, HgSe,HgTe, InAs, InP, GaAs, GaP, ZnO, CdO, HgO, In₂O₃, TiO₂, or a rare earthoxide.

In yet another aspect, for example, this method can be used forpreparing doped core/multiple shell nanocrystals wherein

a) the nanocrystals comprise a core material, a first shell material,and a second material; and have the formula M¹ _(a-c)M² _(c)E¹ _(b)/M³_(d-f)M⁴ _(f)E³ _(e)/M⁵ _(g-i)M⁶ _(i)E⁵ _(h); and

b) the nanocrystals comprise ZnSe/Zn_(d-f)M⁴ _(f)Se/ZnSe,ZnSe/Zn_(d-f)M⁴ _(f)Se/ZnS, ZnO/Zn_(d-f)M⁴ _(f)O/ZnO, ZnO/Zn_(d-f)M⁴_(f)O/ZnS, TiO₂/Ti_(d-f)M⁴ _(f)O₂/TiO₂, and wherein M⁴ is selected fromMn, Fe, Co, Ni, Pd, Pt, Cu, Al, Ag, or Au, or a rare earth metal.

While not intending to be bound by the following statement, it isbelieved that the methods disclosed herein are effective because thereactivity of the precursors are weak enough to prevent theirindependent nucleation, but sufficiently strong to promote the epitaxialgrowth around the existing core nanocrystals. Therefore, relativelystable and less reactive precursors are expected to be more suited forthe growth of high quality core/shell nanocrystals than the traditionalorganometallic precursors, such as dimethyl cadmium, dimethyl zinc andtrimethylsilane sulfide. (See: Qu, L.; Peng, Z. A.; Peng, X. Nano Lett.2001, 1, 333, which is incorporated by reference herein, in itsentirety.)

An additional advantage of this invention is that the relatively stableand less reactive precursors are also relatively inexpensive, more safe,and more environmentally benign, as compared to the traditionalorganometallic precursors. These methods also employ non-coordinatingsolvents which not only provide the necessary tunability of thereactivity of the monomers by varying the ligand concentration insolution, but are also more environmentally safe.

Thus, in one aspect, this invention provides a composition comprisingnanocrystalline, core/shell quantum shells, wherein:

the quantum shells comprise a core material and a shell materialovercoating the core material;

the core material comprises a stable, nanometer-sized inorganic solid;

the shell material overcoating the core material is selected from aII/VI compound or a III/V compound;

the band gap of the core material is greater than the band gap of theshell material;

the thickness of the shell material is from 1 to about 15 monolayers;and

the as-prepared quantum shells having the shell thickness greater than 1monolayer exhibit a photoluminescence that is substantially limited to abandgap emission, with a photoluminescence quantum yield (PL QY) up toabout 20%.

In another aspect, the core material can comprise a II/VI compound or aIII/V compound. In yet another aspect, the nanocrystalline, core/shellquantum shells of this invention are characterized by photogeneratedexcitons that are substantially localized in the shell of the quantumshells. In still another aspect, the quantum shells can exhibit a type-Iband offset. In a further aspect, the core of the quantum shells cancomprise an insulator.

Still another aspect of this invention is a quantum shell that cancomprise CdS/CdSe, CdS/InP, CdS/CdTe, ZnS/CdS, ZnS/CdSe, ZnS/ZnSe,ZnS/CdTe, ZnSe/CdSe, ZnSe/InP, CdS/InAs, CdSe/InAs, ZnSe/InAs, ZnS/InAs,InP/InAs, or a mixture thereof.

In another aspect, the quantum shells of this invention canphotoluminesce at a wavelength from about 400 to about 1000 nm. In thisaspect, the photogenerated excitons are typically radially quantumconfined in the shell of the quantum shells. Further, in this aspect,the quantum shells can exhibit a photoluminescence emission linecharacterized by a FWHM of about 60 nm or less, about 55 nm or less,about 50 nm or less, about 45 nm or less, about 40 nm or less, about 35nm or less, about 30 nm or less, about 28 nm or less, or about 25 nm orless.

In another aspect, the quantum shells of this invention aresubstantially monodisperse. In this aspect, the quantum shells can becharacterized by a size distribution having a standard deviation nogreater than about 15% of a mean diameter of the population of quantumshells, no greater than about 12% of a mean diameter of the populationof quantum shells, no greater than about 10% of a mean diameter of thepopulation of quantum shells, no greater than about 7% of a meandiameter of the population of quantum shells, or no greater than about5% of a mean diameter of the population of quantum shells.

In another aspect, this invention provides devices comprising thequantum shells and compositions of this invention, including, but notlimited to, light-emitting diodes, biological labeling agents,photoelectric devices, lasers, and the like.

In still another aspect, this invention provides a compositioncomprising nanocrystalline, core/shell/shell quantum wells, wherein:

the quantum wells comprise a core material, a first shell materialovercoating the core material, and a second shell material overcoatingthe first shell material; the core material comprises a stable,nanometer-sized inorganic solid;

the first shell material and the second shell material are independentlyselected from a II/VI compound or a III/V compound;

the band gap of the first shell material is less than the band gap ofthe core material and less than the band gap of the second shellmaterial; and

the as-prepared quantum wells exhibit a photoluminescence that issubstantially limited to a bandgap emission, with a photoluminescencequantum yield (PL QY) up to about 50%.

In this aspect, the core material can comprise a II/VI compound or aIII/V compound. Further, the core material of the quantum wells cancomprise an insulator.

In another aspect, for example, the quantum wells of this invention cancomprise CdS/CdSe/CdS, CdS/CdSe/ZnSe, CdS/CdSe/ZnS, ZnSe/CdSe/CdS,ZnSe/CdSe/ZnSe, ZnSe/CdSe/ZnS, ZnS/CdSe/ZnS, ZnS/CdSe/CdS,ZnS/CdSe/ZnSe, ZnS/CdS/ZnS, ZnS/ZnSe/ZnS, CdS/InP/CdS, CdS/InP/ZnSe,CdS/InP/ZnS, ZnSe/InP/ZnSe, ZnSe/InP/CdS, ZnSe/InP/ZnS, ZnS/InP/ZnS,ZnS/InP/CdS, ZnS/InP/ZnSe, CdS/InAs/CdS, CdS/InAs/ZnSe, CdS/InAs/ZnS,ZnSe/InAs/ZnSe, ZnSe/InAs/CdS, ZnSe/InAs/ZnS, ZnS/InAs/ZnS,ZnS/InAs/CdS, ZnS/InAs/ZnSe, CdSe/InAs/CdSe, CdSe/InAs/CdS,CdSe/InAs/ZnS, CdSe/InAs/ZnSe, InAs/InP/CdS, or a mixture thereof. Thus,in another aspect, the core material and the second shell material arethe same.

In another aspect, this invention provides devices comprising thequantum wells and compositions of this invention, including, but notlimited to, light-emitting diodes, biological labeling agents,photoelectric devices, lasers, and the like.

In yet another aspect, this invention provides a composition comprisingnanocrystalline, core/multiple shell quantum wells, wherein:

the quantum wells comprise a core material, a first shell materialovercoating the core material, a second shell material overcoating thefirst shell material, and optionally comprising additional shellmaterials sequentially overcoating underlying shells;

the core material comprises a stable, nanometer-sized inorganic solid;

the first shell material and the second shell material are independentlyselected from a II/VI compound or a III/V compound;

any additional shells are selected from the first shell material and thesecond shell material such that the composition of the additional shellsalternates between the first shell material and the second shellmaterial; and

the band gap of the first shell material is less than the band gap ofthe core material and less than the band gap of the second shellmaterial.

In this aspect, the core material can comprise a II/VI compound or aIII/V compound. Further in this aspect, the core material of the quantumwells can comprise an insulator. In yet another aspect, the as-preparedquantum wells can exhibit a photoluminescence that is substantiallylimited to a bandgap emission, with a photoluminescence quantum yield(PL QY) up to about 10%, up to about 20%, up to about 30%, up to about40%, up to about 50%, or up to about 60%.

In another aspect, this invention provides a composition comprisingradially-doped (or, simply “doped”), core/shell/shell nanocrystalswherein:

the radially-doped nanocrystals comprise a core material, a first shellmaterial overcoating the core material, and a second shell materialovercoating the first shell material;

the core material comprises a compound of the formula M¹ _(x)E_(y),wherein M¹ is selected from a metal, E is selected from a non-metal, andx and y are dictated by the stoichiometry of the compound;

the first shell material comprises a compound of the formula M¹ _(x-z)M²_(z)E_(y), wherein M² is selected from a transition metal or a mixturethereof, 0≦z<x, and M² is different than M¹; and

the second shell material comprises a compound of the formula M¹_(x-q)M³ _(q)E_(y), wherein M³ is selected from a transition metal, arare earth metal, or a mixture thereof, 0≦q≦x, and x is not equal to qwhen M² is the same as M³.

In another aspect, the doped nanocrystals of this invention can becharacterized by:

a) i) M¹ is selected from Zn, Cd, or Hg, and E is selected from O, S.Se, or Te; or

-   -   ii) M¹ is selected from Ga and In, and E is selected from N, P        and As; and

b) M² is selected from Mn, Fe, Co, Ni, Pd, Pt, Cu, Al, Ag, or Au, or arare earth metal.

In another aspect of the doped nanocrystals of this invention, M¹_(x)E_(y) can be selected from CdSe, CdS, CdTe, ZnS, ZnSe, ZnTe, HgS,HgSe, HgTe, InAs, InP, GaAs, GaP, ZnO, CdO, HgO, In₂O₃, TiO₂, or a rareearth oxide. In another aspect of the doped nanocrystals of thisinvention, the radially-doped, core/shell/shell nanocrystals compriseZnSe/Zn_(x-z)M² _(z)Se/ZnSe, ZnSe/Zn_(x-z)M² _(z)Se/ZnS, ZnO/Zn_(x-z)M²_(z)O/ZnO, ZnO/Zn_(x-z)M² _(z)O/ZnS, TiO₂/Ti_(x-z)M² _(z)O₂/TiO₂, andwherein M² is selected from Mn, Fe, Co, Ni, Pd, Pt, Cu, Al, Ag, or Au,or a rare earth metal.

In a further aspect, this invention provides a composition comprisingcore/shell/shell dual-emitting nanocrystals, wherein:

the nanocrystals comprise a core material, a first shell materialovercoating the core material, and a second shell material overcoatingthe first shell material, each of which is independently selected from aII/VI compound or a III/V compound;

the band gap of the first shell material is greater than the band gap ofthe core material and greater than the band gap of the second shellmaterial; and

the as-prepared dual-emitting nanocrystals exhibit a photoluminescencecomprising bandgap emission peaks.

In one aspect, the dual-emitting nanocrystals can further comprise atleast one additional shell material sequentially overcoating underlyingshells, wherein:

the additional shell materials are independently selected from a II/VIcompound or a III/V compound; and

the band gap of the additional shell materials are greater than the bandgap of the second shell material.

In this aspect, the dual-emitting nanocrystals can further comprise upto about 15 additional shells, comprising shell materials wherein:

the additional shell materials are independently selected from selectedfrom a II/VI compound or a III/V compound; and

the band gap of the additional shell materials are greater than the bandgap of the second shell material.

In a further aspect, the present invention provides a compositioncomprising core/shell/shell/shell dual-emitting nanocrystals, wherein:

the nanocrystals comprise a core material, a first shell materialovercoating the core material, a second shell material overcoating thefirst shell material, and a third shell material overcoating the secondshell material, each of which is independently selected from a II/VIcompound or a III/V compound;

the band gap of the first shell material and the band gap of the thirdshell material are less than the band gap of the core material and areless than the band gap of the second shell material; and

the as-prepared dual-emitting nanocrystals exhibit a photoluminescencecomprising two bandgap emissions.

In yet another invention, this invention provides a compositioncomprising core/multiple shell nanocrystals, wherein:

the nanocrystals comprise a core material, a first shell materialovercoating the core material, a second shell material overcoating thefirst shell material, a third shell material overcoating the secondshell material, a fourth shell material overcoating the third shellmaterial, and optionally additional shells overcoating underlyingshells, each of which is independently selected from a II/VI compound ora III/V compound;

the band gap of any shell material is less than the band gap of the bothadjacent core or shell materials, or greater than the band gap of theboth adjacent core or shell materials.

In another aspect of this invention, the present invention provides amethod for preparing radially-doped core/shell/shell nanocrystals havingthe formula M¹ _(x)E_(y)/M¹ _(x-z)M² _(z)E_(y)/M¹ _(x-q)M³ _(q)E_(y),comprising:

a) providing a solution of core nanocrystals of the formula M¹_(x)E_(y), wherein M¹ is selected from a metal, E is selected from anon-metal, and x and y are dictated by the stoichiometry of thecompound;

b) forming at least one monolayer of a doped first shell material of theformula M¹ _(x-z)M² _(z)E_(y) by contacting the core nanocrystals, in analternating manner, with a cation precursor solution in an amounteffective to form a monolayer of the first cation doped with the secondcation, and a first anion (X²) precursor solution in an amount effectiveto form a monolayer of the first anion;

-   -   wherein the cation precursor solution comprises a first cation        (M¹) precursor, a second cation (M²) precursor, or a combination        thereof, and    -   wherein M² is selected from a transition metal or a mixture        thereof, 0≦z<x, and M² is different than M¹;

c) forming at least one monolayer of a second shell material of theformula M¹ _(x-q)M³ _(q)E_(y) by contacting the core/shell nanocrystals,in an alternating manner, with a first cation precursor solution in anamount effective to form a monolayer of the first cation, and an secondanion (X³) precursor solution in an amount effective to form a monolayerof the first anion;

-   -   wherein the first cation precursor solution optionally comprises        a third cation (M³) precursor selected from a transition metal,        a rare earth metal, or a mixture thereof; and    -   wherein 0≦q≦x, and x is not equal to q when M² is the same as        M³; and

d) optionally repeating steps b and c to form additional shellsovercoating the second shell.

In another aspect of this method, M¹E can be selected from a II/VIcompound or a compound. In yet another aspect, q is 0, therefore in thisaspect, the core material and the second shell material have the formulaM¹ _(x)E_(y).

Definitions

In order to more clearly define the terms used herein, the followingdefinitions are provided. To the extent that any definition or usageprovided by any document incorporated herein by reference conflicts withthe definition or usage provided herein, the definition or usageprovided herein controls.

As used herein, the term II/VI compound or II/VI material refers to acompound comprising a group II element (also referred to as a group 12element) and a group VI element (also referred to as group 16 element).In this aspect, the group II elements are Zn, Cd, or Hg; and the groupVI elements are O, S. Se, or Te.

Similarly, as used herein, the term III/V compound or III/V materialrefers to a compound comprising a group III element (also referred to asa group 13 element) and a group V element (also referred to as group 15element). In this aspect, the group III elements are B, Al, Ga, In, orTl; and the group VI elements are N, P, As, Sb, and Bi.

As used herein transition metals include, but not limited to, Ti, Zr,Hf, V, Nb, Ta, Cr, Mo, W, Mn, Tc, Re, Fe, Ru, Os, Co, Rh, Ir, Ni, Pd,Pt, Cu, Ag, and Au. (See, F. A. Cotton et al., Advanced InorganicChemistry, 6th Edition, (1999).) As used herein, including in thecontext of providing core materials, Sc, Y, and La are also consideredtransition metals.

As used herein, the term “stable” refers to both thermal and chemicalstability, in the context of stability under the reactions conditionsused to prepared the nanocrystals of the present invention. Thus,stability to oxidation; reduction; reaction with acids; reaction withbases; electron transfer reactions; internal bond-making; bond-breaking;rearrangement reactions; or any other type of internal reactions orreactions with external reagents are included in this definition. Inthis aspect, for example, the nanometer-sized inorganic core materialsmay be referred to as comprising a stable, nanometer-sized inorganicsolid which refers to their being thermally and chemically stable underthe reaction conditions used to prepare nanocrystals of the presentinvention.

The term “as-prepared”, as used herein, generally refers to samples ofnanocrystals that are used without purification or size-selectionmethods. Specifically, “as-prepared” nanocrystals or nanocrystallineproducts refer to those nanocrystal samples dissolved in the originalreaction mixture or diluted by a solvent without the removal of anyunreacted reactants and the side products.

The term rare-earth or rare-earth metal, as used herein, refers to thelanthanides, the actinides, and Sc, Y, and La. Thus, in this aspect, Sc,Y, and La are encompassed by the terms transition metal and rare-earthmetal.

EXAMPLES

The present invention is further illustrated by the following examples,which are not to be construed in any way as imposing limitations uponthe scope thereof. On the contrary, it is to be clearly understood thatresort may be had to various other embodiments, modifications, andequivalents thereof which, after reading the description herein, maysuggest themselves to one of ordinary skill in the art without departingfrom the spirit of the present invention or the scope of the appendedclaims.

In the following examples, cadmium oxide (CdO, 99.99%), selenium (Se,99.5%, 100 mesh), sulfur (S, 99.98%, powder), trioctylphosphine oxide(TOPO, 90%), tributylphosphine (TBP, 97%), 1-octadecene (ODE), oleicacid (OA, 90%) and octadecylamine (ODA, 97%) were each purchased fromAldrich and were used without further purification. Stearic acid (99%)was obtained from Avocado and was used as received. All organic solventswere purchased from EM Sciences and were likewise used directly withoutany further purification.

Example 1 Synthesis of CdSe Core Nanocrystals

Highly fluorescent CdSe nanocrystals were prepared as described in thefollowing typical reaction. A mixture of 0.2 mmol (25.6 mg) of CdO, 0.8mmol (227 mg) of stearic acid, and 2 g of ODE was prepared in a 25 mL,three-necked flask, and this mixture was heated to about 200° C. to forma clear, colorless solution. The solution was then cooled to roomtemperature, and 1.5 g of ODA and 0.5 g of TOPO were subsequently added.Under an argon flow, this mixture was reheated to 280° C. A seleniumstock solution, prepared by dissolving 2 mmol (158 mg) of Se in 0.472 gof TBP and diluting the solution with 1.37 g of ODE, was quicklyinjected at this injection temperature of 280° C. The mixture was thenmaintained at the growth temperature of 250° C. until the desired sizeof the nanocrystals was obtained.

This reaction generated CdSe nanocrystals of about 3.5 nm in size,characterized by having a first absorption peak around 570 nm. Thereaction mixture was allowed to cool to room temperature, and theresulting nanocrystals were purified from side products and unreactedprecursors using the following extraction procedure. A large volume ofmethanol (at least 3 times of the volume of the reaction mixture) and asmall volume of hexanes (about equal volume of the reaction mixture)were mixed with the reaction mixture, and the unreacted precursors andexcess amines were extracted into the methanol layer. The particleconcentration of the purified CdSe solution in hexanes, as stocksolution for core/shell growth, was measured by using Beer's law withthe extinction coefficients of CdSe nanocrystals. Extinctioncoefficients of various nanocrystals as a function of nanocrystal size,including CdSe nanocrystals, are presented in FIG. 16. The TOPO-coatedCdSe nanocrystals were synthesized using the procedure described inAldana, J.; Wang, Y.; Peng, X. J. Am. Chem. Soc. 2001, 123, 8844, whichis incorporated herein by reference in its entirety.

Example 2 Synthesis of Cadmium (Cation Precursor) and Sulfur (AnionPrecursor) Injection Solutions

Synthesis of the cadmium sulfide shell material for various core/shellnanocrystals, for example, the CdSe/CdS core/shell nanocrystals, wasaccomplished at relatively low temperatures by using the followinginjection solutions for the cation precursor cadmium and the anionprecursor sulfur, respectively. The cadmium injection solution (0.04 M)was prepared by dissolving CdO (0.615 g) in oleic acid (OA, 10.83 g),and 1-octadecene (ODE, 108 mL), at 250° C. The sulfur injection solution(0.04 M) was prepared by dissolving sulfur in ODE at 200° C. Bothinjection solutions were prepared under an argon flow. After clearsolutions were obtained, the cadmium injection solution was allowed tocool to about 60° C. for use, while the sulfur injection solution wasallowed to cool to room temperature for use. For each injection, acalculated amount of a given injection solution was taken with a syringeusing standard air-free procedures.

Example 3 Calculations for the Injection Procedure Using the Cadmium andSulfur Injection Solutions

The successive ionic layer adsorption and reaction (SILAR) techniqueemployed herein, also termed the solution atomic layer epitaxy (SALE)technique, was based on the alternating injections of the cationinjection solution and the anion injection solution, into a solutioncontaining the core nanocrystals. For example, as applied to the growthof CdSe/CdS core/shell nanocrystals, the SILAR technique disclosedherein was based on the alternating injections of the Cd injectionsolution and the S injection solution, into a solution containing CdSenanocrystals.

The amount of cadmium or sulfur, and hence the amount of injectionsolution, needed for each layer was determined by calculating the numberof surface atoms of a given sized core/shell nanocrystal. Since there isonly about a 5-6% lattice mismatch between CdSe and CdS bulk crystals,the calculations were based on the wurtzite structure of CdSenanocrystals. On average, the thickness of one monolayer of CdS wasassumed as 0.35 nm, so one additional layer growth would increase thesize of the nanocrystals by about 0.7 nm in diameter. For example, in atypical experiment with 1×10⁻⁵ mmol of 3.7 mm core CdSe particles,2.13×10⁻³ mmol of the Cd and S precursors are needed for the first layerof the shell growth, and additional 2.85×10⁻³ mmol of the Cd and Sprecursors will complete the growth of the second layer.

Example 4 Synthesis of CdSe/CdS Core/Shell Nanocrystals Using theSuccessive Ionic Layer Adsorption and Reaction (SILAR) Technique

In a typical successive ionic layer adsorption and reaction (SILAR)technique preparation of CdSe/CdS core/shell nanocrystals, alternatinginjections of the Cd and S injection solutions into a CdSenanocrystal-containing solution were carried out. A stock solution of2.44 g of CdSe nanocrystals in hexanes, 3.5 nm in diameter and 1.01×10⁻⁴mmol in hexanes, was mixed with 1.5 g of ODA and 5.0 g of ODE in a 25mL, three-necked flask. The flask was then sealed and placed undervacuum at room temperature for 30 min to remove the hexanes, then heatedto 100° C. while still under vacuum for another 5-10 min to remove anyresidual air from the system. After this procedure, the CdSenanocrystal-containing solution was placed under an argon flow and thereaction mixture was further heated to 240° C. in preparation forinjections. The first injection was made using 0.49 mL of the Cdinjection solution (0.04 M). The amounts of the injection solutions werecalculated for each subsequent injection using the method describedabove. To monitor the reaction, aliquots were taken at the first minuteafter injection and subsequently every 3-5 min, and absorptionmeasurements of each aliquot were examined. If there was no observablechange in the UV-Vis absorption spectrum for two successive aliquots 3-5min separated, the injection of another shell component for either thesame layer (nS) or the next layer ((n+1)Cd) was executed.

After no further UV-Vis peak shift was observed in 5 min following thefifth sulfur injection, the reaction was terminated by allowing thereaction mixture to cool to room temperature. The final product waspurified by diluting the reaction solution with hexanes, and thecontaminants were extracted from the hexane solution with methanol.Alternatively, the resulting nanocrystals were precipitated by addingacetone to the hexanes solution. Excess amines were further removed bydissolving the nanocrystals in chloroform and precipitating them withacetone.

The reaction temperature was varied between 120° C. and 260° C. in orderto study the temperature effect for the growth of CdSe/CdS core/shellnanocrystals. For these reactions at different temperatures, the otherreaction conditions and procedures were the same as the typicalsynthesis.

Example 5 Multigram-Scale Synthesis of CdSe/CdS Core/Shell NanocrystalsUsing the Successive Ionic Layer Adsorption and Reaction (SILAR)Technique

The SILAR procedure described above was readily extended to a largescale, multi-gram synthesis. In a typical large scale synthesis, 195 gof CdSe nanocrystal stock solution (3.5 nm in size, 5.87×10⁻³ mmol ofparticles in hexanes) was mixed with 250 g of ODE and 75 g of ODA in athree-necked flask. After removal of hexanes and air under vacuum, themixture was heated to 240° C. in an argon atmosphere, for the cationprecursor and anion precursor injection and nanocrystal growth steps.With the same injection solutions used in the small scale synthesis, theamounts of the injection solutions injected for each step were asfollows: 1) 27 mL of the Cd and S solution for the first Cd and Sinjection; 2) 37 mL of each injection solution for the second layer; 3)48.6 mL of each injection solution for the third layer; 4) 61.8 mL ofeach injection solution for the fourth layer; and 5) 76.6 mL of eachinjection solution for the fifth layer. The reaction solution wasrapidly stirred during injection, therefore is was not necessary toinject the Cd and S injection solutions in a dropwise manner. After thislarge scale reaction, the raw products were separated by acetoneprecipitation followed by centrifugation. The sample of argon-flow driednanocrystals, after purification, weighed about 2.5 grams.

Example 6 Optical Measurements of the CdSe/CdS Core/Shell Nanocrystals

Absorption spectra were measured using an HP 8453 diode arrayspectrophotometer. Photoluminescence (PL) and photoluminescenceexcitation (PLE) spectra were examined using a HITACHI F-2500fluorescence spectrophotometer. The photoluminescence (PL) quantum yield(QY) of samples was obtained using organic dyes with known quantumyields as the standard. Suitable methods and examples of this procedureare disclosed in: Peng, X.; Schlamp, M. C.; Kadavanich, A. V.;Alivisatos, A. P. J. Am. Chem. Soc. 1997, 119, 7019-7029; Manna, L.;Scher, E. C.; Li, L.-S.; Alivisatos, A. P. J. Am. Chem. Soc. 2002, 124,7136-7145; Qu, L.; Peng, X. J. Am. Chem. Soc. 2002, 124, 2049-2055; eachof which is incorporated herein by reference in its entirety.

Careful studies revealed that the PL QY of colloidal semiconductornanocrystals determined by typical procedures varied somewhat when usingdifferent organic dyes as the PL QY standard. Accordingly, the PL QYdata reported herein were obtained using R640 as the standard, whichprovided significantly lower values than those obtained when using otherdyes, such as LD690. Both organic dyes (R640 and LD690) were purchasedfrom Exciton.

Example 7 Transmission Electron Microscopy (TEM) Images of Nanocrystals

Low resolution TEM images of the nanocrystals described herein, forexample, the CdSe/CdS core/shell nanocrystals, were obtained using aJOEL 100CX transmission electron microscope with 80 kV accelerationvoltage. The CdSe/CdS core/shell nanocrystals samples examined weretypically purified by either acetone precipitation from a chloroformsolution, or hexanes/methanol extraction. Formvar® film-coated coppergrids were dipped into hexanes or toluene solutions of the nanocrystalsto deposit the nanocrystals onto the film. Randomly orientednanocrystals on the TEM substrate were obtained by using a dilutednanocrystal solution, with an absorbance of the first absorption peak ofthe nanocrystals below approximately 0.05. If the absorbance of thefirst absorption peak was above about 0.2, densely packed monolayers andmultilayers of nanocrystals were observed. Selected area electrondiffraction patterns (SAED) were obtained with a camera length of 120cm.

High resolution TEM pictures were obtained using a JEOL 2000 microscope,in which the nanocrystals were deposited onto thin carbon gridspurchased from Ted Pella, Inc., Redding, Calif.

Example 8 X-Ray Powder Diffraction (XRD) Images of Nanocrystals

X-ray Powder Diffraction (XRD) patterns of the nanocrystals describedherein, for example, the CdSe/CdS core/shell nanocrystals, were obtainedusing a Philips PW1830 X-ray diffractometer. In order to obtainmeaningful diffraction patterns for the CdSe/CdS core/shellnanocrystals, the excess ODA ligands were thoroughly removed from asample of the nanocrystals.

Example 9 Photo-Oxidation Experiments on Nanocrystals

The photo-oxidation tests on the nanocrystals described herein, forexample, on the CdSe/CdS core/shell nanocrystals, were performed usingeither the standard method described in Aldana, J.; Wang, Y.; Peng, X.J. Am. Chem. Soc. 2001, 123, 8844 and Wang, Y. A.; Li, J. J.; Chen, H.;Peng, X. J. Am. Chem. Soc. 2002, 124, 2293-2298, or using a CoherentInc. argon ion laser with 50-100 mW of a 514 nm beam, as described inManna, L.; Scher, E. C.; Li, L.-S.; Alivisatos, A. P. J. Am. Chem. Soc.2002, 124, 7136-7145, each of which is incorporated herein by referencein its entirety.

For the laser-related experiments, a light beam spot, 0.8 cm indiameter, was projected onto a sample by inserting a diverging lens inthe beam path. Nanocrystal solutions, saturated with oxygen or argon,were compared under the same testing conditions. Photoluminescencespectra were measured before and after irradiation.

Example 10 Thin Film Deposition of CdSe Plain Core and CdSe/CdSCore/Shell Nanocrystals

Plain core nanocrystals or core/shell nanocrystals were deposited ontoglass or ITO (indium-doped tin oxide) substrates by spin coating thenanocrystals from toluene solutions. The PL spectra of the thin filmswere taken by placing the substrates at the cross-point of theexcitation beam and the normal direction of the detector, with a 30°angle relative to both directions. The PL spectra of the two originalsolutions with the same absorbance at the excitation wavelength werecompared. The PL spectra of the two thin films of CdSe nanocrystals andCdSe/CdS core/shell nanocrystals were compared by normalizing the PLintensity using the absorbance of thin films at the excitationwavelength.

Example 11 Surface Modification of CdSe Plain Core and CdSe/CdSCore/Shell Nanocrystals with Mercaptopropionic Acid

Surface modification of both the CdSe plain core and CdSe/CdS core/shellnanocrystals with mercaptopropionic acid was performed using thestandard procedure described in Aldana, J.; Wang, Y.; Peng, X. J. Am.Chem. Soc. 2001, 123, 8844. The UV-Vis and PL measurements were carriedout using the purified nanocrystals dissolved in water.

Example 12 Synthesis of CdSe/ZnS Core/Shell Nanocrystals Using theSuccessive Ionic Layer Adsorption and Reaction (SILAR) Technique

In a typical successive ionic layer adsorption and reaction (SILAR)technique preparation of CdSe/ZnS core/shell nanocrystals, alternatinginjections of the Zn and S injection solutions into a CdSenanocrystal-containing solution were carried out. A stock solution of2.44 g of CdSe nanocrystals in hexanes, 3.5 nm in diameter and 1.01×10⁻⁴mmol in hexanes, was mixed with 1.5 g of ODA and 5.0 g of ODE in a 25mL, three-necked flask. The flask was then sealed and placed undervacuum at room temperature for 30 min to remove the hexanes, then heatedto 100° C. while still under vacuum for another 5-10 min to remove anyresidual air from the system. After this procedure, the CdSenanocrystal-containing solution was placed under an argon flow and thereaction mixture was further heated to 270° C. in preparation forinjections. The first injection was made using 0.49 mL of the Zninjection solution (0.04 M). The amounts of the injection solutions werecalculated for each subsequent injection using the method describedabove. To monitor the reaction, aliquots were taken at the first minuteafter injection and subsequently every 3-5 min, and absorptionmeasurements of each aliquot were examined. If there was no observablechange in the UV-Vis absorption spectrum for two successive aliquots 3-5min separated, the injection of another shell component for either thesame layer (nS) or the next layer ((n+1)Zn) was executed.

After no further UV-Vis peak shift was observed in 5 min following thefifth sulfur injection, the reaction was terminated by allowing thereaction mixture to cool to room temperature. The final product waspurified by diluting the reaction solution with hexanes, and thecontaminants were extracted from the hexane solution with methanol.Alternatively, the resulting nanocrystals were precipitated by addingacetone to the hexanes solution. Excess amines were further removed bydissolving the nanocrystals in chloroform and precipitating them withacetone.

Example 13 Synthesis of InAs/CdS Core/Shell Nanocrystals Using theSuccessive Ionic Layer Adsorption and Reaction (SEAR) Technique

Plain InAs core nanocrystals (or simply, “cores”) were synthesized usinga method analogous to that described in Battaglia, D; Peng, X.NanoLetters 2002, 2(9), 1027, which is incorporated herein by referencein its entirety. Once the core InAs nanocrystals had grown tocompletion, cadmium injection solutions (CdO-oleic acid in ODE) andsulfur injection solutions (sulfur in ODE), prepared as detailed in theExamples herein, were added dropwise to the solution containing the InAscores at about 250° C., according to a similar method as that used forthe CdSe/CdS core/shell nanocrystals described herein. This synthesismethod initially formed an alloyed layer of InCdS for the first fewlayers, but then became pure CdS for the layers subsequent to that.

Example 14 Synthesis of InAs/InP Core/Shell Nanocrystals Using theSuccessive Ionic Layer Adsorption and Reaction (SEAR) Technique

Plain InAs core nanocrystals (cores) were synthesized using a methodanalogous to that described in Battaglia, D; Peng, X. NanoLetters 2002,2(9), 1027, which is incorporated herein by reference in its entirety.Once the core InAs nanocrystals finished growing, the reaction solutionwas then cooled to 250° C. from 300° C. andtris(trimethylsilyl)phosphine in 1 gram octadecene (ODE) (0.05 mmol) wasadded to the reaction solution. The tris(trimethylsilyl)phosphineprecursor reacted with the excess indium already in the solution, due tothe 8:1 indium to arsenic molar ratio used for the InAs core nanocrystalformation, to form the InP layers. More tris(trimethylsilyl)phosphineprecursor could be added in the same manner to increase the InP shellthickness. This one-pot, core/shell method enabled InP to be grown onthe surface of the InAs nanocrystals without exposing them to the air,thus stabilizing the InAs nanocrystals from oxidation.

Example 15 Synthesis of CdS/CdSe Quantum Shell Nanocrystals Using theSuccessive Ionic Layer Adsorption and Reaction (SILAR) Technique

A sample of CdS core nanocrystals of a known size and concentration wasused to prepare a CdS reaction solution containing ODE as the solventand octadecylamine (ODA) as the ligand. A typical reaction contained5.0×10⁻⁸ mol of CdS, however, the concentration did not need to be fixedas long as the precursor injections were enough to create a singlemonolayer on the surface of the cores. Once the CdS, 5 grams of ODE, and1 gram of ODA were added to the reaction flask the reaction was heatedup to 100° C. while under vacuum. When the reaction stopped bubbling,the solution was placed under an argon flow and heated up to 190° C. forCdSe shell growth. The Cd-Oleic acid and pure Se precursors in ODE (0.04M each) were added in alternating injections at 190° C. The volume ofeach injection was selected to create one monolayer of CdSe on thesurface of the CdS cores. Additional layers could be added by increasingthe injection amounts to take into considerating the increase in sizefrom the shell already grown. (core+0.7 nm).

Example 16 Synthesis of CdS/InP Quantum Shell Nanocrystals Using theSuccessive Ionic Layer Adsorption and Reaction (SILAR) Technique

A sample of CdS core nanocrystals of a known size and concentration wasused to prepare a CdS reaction solution containing ODE as the solventand octadecylamine (ODA) as the ligand. A typical reaction contained5.0×10⁻⁸ mol of CdS, however, the concentration did not need to be fixedas long as the precursor injections were enough to create a singlemonolayer on the surface of the cores. Once the CdS, 5 grams of ODE, and1 gram ODA were added to the reaction flask, the reaction was heated upto 100° C. while under vacuum. When the reaction stopped bubbling, thesolution was placed under an argon flow and heated up to 180° C. for InPshell growth. The indium-Oleic acid and tris(trimethylsilylphosphine)precursors in ODE (0.04 M each) were added in alternating injections at180° C. The volume of each injection was selected to create onemonolayer of InP on the surface of the CdS cores.

Example 17 Synthesis of CdS/CdSe/CdS Quantum Well Nanocrystals Using theSuccessive Ionic Layer Adsorption and Reaction (SILAR) Technique

A sample of CdS core nanocrystals of a known size and concentration wasused to prepare a CdS reaction solution containing ODE as the solventand oleic acid (OA) as the ligand. Oleic acid was used because it is agood ligand with respect to sustaining good PL emission properties forCdS, but not for CdSe. A typical reaction contained 5.0×10⁻⁸ mol of CdS,however, the concentration did not need to be fixed as long as theprecursor injections were enough to create a single monolayer on thesurface of the cores. Once the CdS, 5 grams of ODE, and 1 gram of oleicacid were added to the reaction flask, the reaction was heated up to100° C. while under vacuum. When the reaction stopped bubbling, thesolution was put under an argon flow and heated up to 200° C. for theCdSe shell growth and 240° C. for the CdS growth. The Cd-Oleic acid andpure Se precursors in ODE (0.04 M each) were added in alternatinginjections at 200° C. The volume of each injection was selected tocreate one monolayer of CdSe on the surface of the CdS cores. Thereaction temperature was then increased to 240° C., and alternatinginjections of Cd-Oleic acid and pure S in ODE (0.04 M each) were addedto create the monolayers of CdS on the CdS/CdSe core/shell nanocrystals,and hence form the CdS/CdSe/CdS core/shell/shell nanocrystals.

Example 18 Synthesis of Dual-Emitting CdSe/ZnS/CdSe 0D-1D HybridNanocrystals Using the Successive Ionic Layer Adsorption and Reaction(SILAR) Technique

A sample of CdSe core nanocrystals of a known size and concentration wasused to prepare a CdSe reaction solution containing ODE as the solventand octadecylamine (ODA) as the ligand. A typical reaction contained1.25×10⁻⁸ mol of CdSe, however, the concentration did not need to befixed as long as the precursor injections were enough to create a singlemonolayer on the surface of the cores. Once the CdSe, 5 grams of ODE,and 1 gram of ODA were added to the reaction flask, the reaction washeated up to 100° C. while under vacuum. Once the reaction stoppedbubbling, the solution was put under an argon flow and heated up to 200°C. for the ZnS layering. The Zn-Oleic acid and pure S precursors in ODE(0.04 M each) were added in alternating injections at 200° C. in orderto grow a single monolayer at a time of ZnS on the surface of the CdSe.Using this method, many layers of ZnS can be layered on in a controlledfashion. In one aspect, it was found that 2 to 4 monolayers gaveexcellent results. The reaction temperature was then decreased to 190°C. and Cd-Oleic acid and pure Se precursors in ODE (0.04M) were added inalternating injections in order to grow the CdSe monolayers. The CdSecould be layered as many times as needed. No secondary emission was seenfor the first monolayer. In one aspect, the shell was not be layered tothe extent that its emission overlapped that of the core, which wouldotherwise eliminate the detection of the core emission.

Example 19 Synthesis of Doped Nanocrystals Using the Successive IonicLayer Adsorption and Reaction (SILAR) Technique

Pre-synthesized ZnSe nanocrystals (10 mg) were added into a three-neckflask with ODE (3 g) and octadecylamine (2 g). Under the sameexperimental conditions discussed in Example 5, the solution was heatedto about 200-260° C. Either Cu stearate or Mn stearate were used as theprecursor for the Cu and Mn dopants, respectively. The growth of thenanocrystals was achieved by adding the Zn stearate solution and Sesolution as usual SILAR procedures as disclosed herein. The dopants wereadded into the Zn solution as needed and in a concentration required ina given monolayer.

The invention claimed is:
 1. A composition comprising: (a) substantiallymonodisperse core/shell nanocrystals, the core/shell nanocrystalscomprising a core and a first shell overcoating the core, wherein thecore comprises a II/VI compound or a III/V compound, the first shellcomprises 3 or more monolayers of a II/VI compound or a III/V compound,the first shell and the core have different chemical compositions, andthe core/shell nanocrystals have a size distribution having a standarddeviation no greater than about 10% of a mean diameter of thenanocrystals; (b) a solvent; and (c) unreacted reactants and sideproducts.
 2. The composition of claim 1, wherein the first shellcomprises 4 or more monolayers of the II/VI compound or the III/Vcompound.
 3. The composition of claim 1, wherein the first shellcomprises from 3 monolayers to 15 monolayers of the II/VI compound orthe III/V compound.
 4. The composition of claim 1, wherein thecore/shell nanocrystals are as-prepared.
 5. The composition of claim 1,wherein the core/shell nanocrystals have a size distribution having astandard deviation no greater than about 5% of a mean diameter of thenanocrystals.
 6. The composition of claim 1, wherein the bandgap of thecore is less than the bandgap of the first shell.
 7. The composition ofclaim 1, wherein the core/shell nanocrystals exhibit a type-I bandoffset or a type-II band offset.
 8. The composition of claim 1, whereinthe core is selected from the group consisting of CdSe, CdS, CdTe, ZnSe,ZnS, ZnTe, HgSe, HgS, HgTe, ZnO, CdO, GaAs, InAs, GaP and InP; the firstshell is selected from the group consisting of CdSe, CdS, CdTe, ZnSe,ZnS, ZnTe, HgSe, HgS, HgTe, ZnO, CdO, GaAs, InAs, GaP and InP.
 9. Thecomposition of claim 1, wherein the core/shell nanocrystals compriseCdSe/CdS, CdSe/ZnSe, CdSe/ZnS, CdS/ZnS, CdTe/CdSe, CdTe/CdS, CdTe/ZnTe,CdTe/ZnSe, CdTe/ZnS, ZnSe/ZnS, ZnTe/CdS, ZnTe/ZnSe, InAs/InP, InAs/CdSe,InAs/CdS, InAs/ZnS, InP/CdS, InP/ZnS, InP/ZnSe, InAs/InP/CdS or amixture thereof.
 10. The composition of claim 1, wherein the core/shellnanocrystals exhibit a photoluminescence quantum yield (PL QY) rangingfrom about 20% to about 40%.
 11. The composition of claim 1, wherein thecore/shell nanocrystals have a luminescence at a wavelength from about400 nm to about 1000 nm.
 12. The composition of claim 1, wherein thecore/shell nanocrystals exhibit a photoluminescence emission linecharacterized by a FWHM of about 50 nm or less.
 13. The composition ofclaim 1, wherein the core/shell nanocrystals exhibit a photoluminescenceemission line characterized by a FWHM of about 40 nm or less.
 14. Thecomposition of claim 1, wherein the core/shell nanocrystals exhibit aphotoluminescence emission line characterized by a FWHM of about 28 nmor less when the core material comprises CdSe.
 15. The composition ofclaim 1, wherein the first shell of the core/shell nanocrystals is aquantum shell.
 16. The composition of claim 15, wherein photogeneratedexcitons are radially confined to the quantum shell.
 17. The compositionof claim 15, wherein the core of the core/shell nanocrystals comprisesan insulator.
 18. The composition of claim 15, wherein the core/shellnanocrystals comprise CdS/CdSe, CdS/InP, CdS/CdTe, ZnS/CdS, ZnS/CdSe,ZnS/ZnSe, ZnS/CdTe, ZnSe/CdSe, ZnSe/InP, CdS/InAs, CdSe/InAs, ZnSe/InAs,ZnS/InAs, InP/InAs or a mixture thereof.
 19. A composition comprising:(a) core/shell/shell nanocrystals, the core/shell/shell nanocrystalscomprising a first shell overcoating the core and a second shellovercoating the first shell, wherein the core comprises a II/VI compoundor a III/V compound, the first shell and the second shell each comprisea II/VI compound or a III/V compound, the first shell and the core havedifferent chemical compositions, the first shell and the second shellhave different chemical compositions, at least one of the first shelland the second shell comprises 3 or more monolayers of the II/VIcompound or the III/V compound, and the core/shell/shell nanocrystalshave a size distribution having a standard deviation no greater thanabout 10% of a mean diameter of the nanocrystals; (b) a solvent; and (c)unreacted reactants and side products.
 20. The composition of claim 19,wherein the first shell comprises 3 or more monolayers of the II/VIcompound or the III/VI compound.
 21. The composition of claim 19,wherein the second shell comprises 3 or more monolayers of the II/VIcompound or the III/VI compound.
 22. The composition of claim 19,wherein the core/shell/shell nanocrystals are as-prepared.
 23. Thecomposition of claim 19, wherein the core/shell/shell nanocrystals arequantum wells.
 24. The composition of claim 23, wherein the core and thesecond shell comprise the same II/VI compound or III/V compound.
 25. Thecomposition of claim 19, wherein the bandgap of the first shell is lessthan the bandgap of the core and less than the bandgap of the secondshell.
 26. The composition of claim 23, wherein the quantum wellscomprise CdS/CdSe/CdS, CdS/CdSe/ZnSe, CdS/CdSe/ZnS, ZnSe/CdSe/CdS,ZnSe/CdSe/ZnSe, ZnSe/CdSe/ZnS, ZnS/CdSe/ZnS, ZnS/CdSe/CdS,ZnS/CdSe/ZnSe, ZnS/CdS/ZnS, ZnS/ZnSe/ZnS, CdS/InP/CdS, CdS/InP/ZnSe,CdS/InP/ZnS, ZnSe/InP/ZnSe, ZnSe/InP/CdS, ZnSe/InP/ZnS, ZnS/InP/ZnS,ZnS/InP/CdS, ZnS/InP/ZnSe, CdS/InAs/CdS, CdS/InAs/ZnSe, CdS/InAs/ZnS,ZnSe/InAs/ZnSe, ZnSe/InAs/CdS, ZnSe/InAs/ZnS, ZnS/InAs/ZnS,ZnS/InAs/CdS, ZnS/InAs/ZnSe, CdSe/InAs/CdSe, CdSe/InAs/CdS,CdSe/InAs/ZnS, CdSe/InAs/ZnSe, InAs/InP/CdS, or a mixture thereof. 27.The composition of claim 19, wherein the core comprises an insulator.28. The composition of claim 19, wherein the core/shell/shellnanocrystals have a size distribution having a standard deviation nogreater than about 5% of a mean diameter of the nanocrystals.
 29. Thecomposition of claim 23, wherein the quantum wells exhibit aphotoluminescence quantum yield ranging from about 10% to about 50%. 30.The composition of claim 23, wherein the quantum wells exhibit aphotoluminescence quantum yield ranging from about 20% to about 50%. 31.The composition of claim 15, wherein the nanocrystals exhibit aphotoluminescence quantum yield ranging from about 10% to about 20%. 32.The composition of claim 15, wherein the nanocrystals exhibit aphotoluminescence emission line characterized by a FWHM of about 60 nmor less.
 33. The composition of claim 15, wherein the nanocrystalsexhibit a photoluminescence emission line characterized by a FWHM ofabout 40 nm or less.
 34. The composition of claim 15, wherein thenanocrystals exhibit a photoluminescence emission line characterized bya FWHM of about 30 nm or less.
 35. The composition of claim 15, whereinthe nanocrystals have a size distribution having a standard deviation nogreater than about 10% of a mean diameter of the nanocrystals.
 36. Acomposition comprising: (a) substantially monodisperse core/shellnanocrystals, the core/shell nanocrystals comprising a core and a firstshell overcoating the core, wherein the core comprises a II/VI compoundor a III/V compound, the first shell comprises 2 or more monolayers of aII/VI compound or a III/V compound, the first shell and the core havedifferent chemical compositions, and the core/shell nanocrystals have asize distribution having a standard deviation no greater than about 10%of a mean diameter of the nanocrystals; (b) a solvent; and (c) unreactedreactants and side products.
 37. The composition of claim 1, wherein thefirst shell comprises 5 or more monolayers of the II/VI compound or theIII/V compound.